Disable endpoints when disconnecting on Kinetis

Even thought the USB phy is disabled during disconnect, endpoints
still nack packets sent. This patch disables all endpoints so after
disconnect no USB traffic will be sent. This allows brief disconnects
of ~200us (or more) to be reliably detected by the host PC.

targets/TARGET_Freescale/usb/USBPhy_Kinetis.cpp

@@ -195,6 +195,10 @@ void USBPhyHw::connect()
 
 void USBPhyHw::disconnect()
 {
+    // disable all endpoints to prevent them from nacking when disconnected
+    for(int i = 0; i < 16; i++) {
+        USB0->ENDPOINT[i].ENDPT = 0x00;
+    }
     // disable USB
     USB0->CTL &= ~USB_CTL_USBENSOFEN_MASK;
     // Pull up disable