usb: gadget: forbid queuing request to a disabled ep

Queue a request to disabled ep  doesn't make sense, and induce caller
make mistakes.

Here is a example for the android mtp gadget function driver. A mem
corruption can happen on below senario.
1) On disconnect, mtp driver disable its EPs,
2) During send_file_work and receive_file_work, mtp queues a request
   to ep. (The mtp driver need improve its synchronization logic!)
3) mtp_function_unbind is invoked and all mtp requests are freed.
4) when udc process the request queued on step 2, will cause kernel
   NULL pointer dereference exception.
Signed-off-by: Du, Changbin <changbin.du@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

include/linux/usb/gadget.h

@@ -402,6 +402,9 @@ static inline void usb_ep_free_request(struct usb_ep *ep,
 static inline int usb_ep_queue(struct usb_ep *ep,
 			       struct usb_request *req, gfp_t gfp_flags)
 {
+	if (WARN_ON_ONCE(!ep->enabled && ep->address))
+		return -ESHUTDOWN;
+
 	return ep->ops->queue(ep, req, gfp_flags);
 }