Implement endpoint_abort for STM32

Make use of the added function HAL_PCD_EP_Abort to implement
endpoint_abort.

usb/device/targets/TARGET_STM/USBPhy_STM32.cpp

@@ -475,7 +475,8 @@ bool USBPhyHw::endpoint_write(usb_ep_t endpoint, uint8_t *data, uint32_t size)
 
 void USBPhyHw::endpoint_abort(usb_ep_t endpoint)
 {
-    // TODO - stop the current transfer on this endpoint and don't call the IN or OUT callback
+    HAL_StatusTypeDef ret = HAL_PCD_EP_Abort(&hpcd, endpoint);
+    MBED_ASSERT(ret==HAL_OK);
 }
 
 void USBPhyHw::process()