Pairing fails when IPv6 enabled in SoftAP intf

Issue: udp_sendto() Fails for multicast IPV6 packet when interface is switched from SoftAP to STA mode.
When SoftAP start is called,  add_ethernet_interface() will be called internally to add interface details into  netif_list.
When switching from SoftAP to STA mode,  add_ethernet_interface() will be called again to append the interace details into netif_list.
When udp_sendto() is called, ip6_route() will return interface as NULL since it consider device as single interface.
Fix: SoftAP mode Stop, call remove_ethernet_interface() to remove the interface from the netif_list.

features/lwipstack/LWIPInterface.cpp

@@ -465,6 +465,41 @@ nsapi_error_t LWIP::add_ethernet_interface(EMAC &emac, bool default_if, OnboardN
 #endif //LWIP_ETHERNET
 }
 
+nsapi_error_t LWIP::remove_ethernet_interface(OnboardNetworkStack::Interface **interface_out)
+{
+#if LWIP_ETHERNET
+
+    if ((interface_out != NULL) && (*interface_out != NULL)) {
+
+        Interface *lwip = static_cast<Interface *>(*interface_out);
+        Interface *node = lwip->list;
+
+        if (lwip->list != NULL) {
+            if (lwip->list == lwip) {
+                lwip->list = lwip->list->next;
+                netif_remove(&node->netif);
+                *interface_out = NULL;
+                delete node;
+            } else {
+                while (node->next != NULL && node->next != lwip) {
+                    node = node->next;
+                }
+                if (node->next != NULL && node->next == lwip) {
+                    Interface *remove = node->next;
+                    node->next = node->next->next;
+                    netif_remove(&remove->netif);
+                    *interface_out = NULL;
+                    delete remove;
+                }
+            }
+        }
+    }
+
+    return NSAPI_ERROR_OK;
+#else
+    return NSAPI_ERROR_UNSUPPORTED;
+#endif //LWIP_ETHERNET
+}
 
 nsapi_error_t LWIP::add_l3ip_interface(L3IP &l3ip, bool default_if, OnboardNetworkStack::Interface **interface_out)
 {

features/lwipstack/LWIPStack.h

@@ -281,6 +281,14 @@ public:
      * @param[out] interface_out    pointer to stack interface object controlling the L3IP
      * @return                      NSAPI_ERROR_OK on success, or error code
      */
+    virtual nsapi_error_t remove_ethernet_interface(OnboardNetworkStack::Interface **interface_out);
+
+    /** Remove a network interface from IP stack
+     *
+     * Removes PPP objects,network interface from stack list, and shutdown device driver.
+     * @param[out] interface_out    pointer to stack interface object controlling the PPP
+     * @return                      NSAPI_ERROR_OK on success, or error code
+     */
     virtual nsapi_error_t remove_l3ip_interface(OnboardNetworkStack::Interface **interface_out);
 
     /** Remove a network interface from IP stack

features/netsocket/OnboardNetworkStack.h

@@ -181,6 +181,11 @@ public:
         return NSAPI_ERROR_UNSUPPORTED;
     };
 
+    virtual nsapi_error_t remove_ethernet_interface(Interface **interface_out)
+    {
+        return NSAPI_ERROR_OK;
+    };
+
     virtual nsapi_error_t remove_l3ip_interface(Interface **interface_out)
     {
         return NSAPI_ERROR_OK;

features/netsocket/emac-drivers/TARGET_Cypress/COMPONENT_WHD/interface/WhdSTAInterface.cpp

@@ -355,6 +355,15 @@ nsapi_error_t WhdSTAInterface::disconnect()
     }
     whd_emac_wifi_link_state_changed(_whd_emac.ifp, WHD_FALSE);
 
+    // remove the interface added in connect
+    if (_interface) {
+        nsapi_error_t err = _stack.remove_ethernet_interface(&_interface);
+        if (err != NSAPI_ERROR_OK) {
+            return err;
+        }
+        _iface_shared.iface_sta = NULL;
+    }
+
     res = whd_wifi_deregister_event_handler(_whd_emac.ifp, sta_link_update_entry);
     if (res != WHD_SUCCESS) {
         return whd_toerror(res);

features/netsocket/emac-drivers/TARGET_Cypress/COMPONENT_WHD/interface/WhdSoftAPInterface.cpp

@@ -201,6 +201,16 @@ int WhdSoftAPInterface::stop(void)
     if (res != WHD_SUCCESS) {
         return whd_toerror(res);
     }
+
+    // remove the interface added in start
+    if (_interface) {
+        nsapi_error_t err = _stack.remove_ethernet_interface(&_interface);
+        if (err != NSAPI_ERROR_OK) {
+            return err;
+        }
+        _iface_shared.iface_softap = NULL;
+    }
+
     return NSAPI_ERROR_OK;
 }