Inline nsapi_create_stack(NetworkStack)

The rather fiddly `nsapi_create_stack` template + overloads used during
socket formation don't inline their core, which is the identity operation
for `NetworkStack *` itself. Make code generation easier by having that
core be inline.

UNITTESTS/stubs/NetworkStack_stub.cpp

@@ -68,11 +68,6 @@ NetworkStack *nsapi_create_stack(nsapi_stack_t *stack)
     return reinterpret_cast<NetworkStack *>(stack);
 }
 
-NetworkStack *nsapi_create_stack(NetworkStack *stack)
-{
-    return reinterpret_cast<NetworkStack *>(stack);
-}
-
 nsapi_value_or_error_t NetworkStack::gethostbyname_async(const char *host, hostbyname_cb_t callback, nsapi_version_t version,
                                                          const char *interface_name)
 {

features/netsocket/NetworkStack.cpp

@@ -445,8 +445,3 @@ NetworkStack *nsapi_create_stack(nsapi_stack_t *stack)
     return new (stack->_stack_buffer) NetworkStackWrapper;
 }
 
-NetworkStack *nsapi_create_stack(NetworkStack *stack)
-{
-    return stack;
-}
-

features/netsocket/NetworkStack.h

@@ -483,14 +483,18 @@ private:
 
 /** Convert a raw nsapi_stack_t object into a C++ NetworkStack object
  *
- *  @param stack    Reference to an object that can be converted to a stack
+ *  @param stack    Pointer to an object that can be converted to a stack
  *                  - A raw nsapi_stack_t object
- *                  - A reference to a network stack
+ *                  - A pointer to a network stack
- *                  - A reference to a network interface
+ *                  - A pointer to a network interface
- *  @return         Reference to the underlying network stack
+ *  @return         Pointer to the underlying network stack
  */
 NetworkStack *nsapi_create_stack(nsapi_stack_t *stack);
-NetworkStack *nsapi_create_stack(NetworkStack *stack);
+
+inline NetworkStack *nsapi_create_stack(NetworkStack *stack)
+{
+    return stack;
+}
 
 template <typename IF>
 NetworkStack *nsapi_create_stack(IF *iface)