usb: gadget: udc: core: update usb_ep_queue() documentation

Mention that ->complete() should never be called from within
usb_ep_queue().

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/core.c

@@ -238,6 +238,9 @@ EXPORT_SYMBOL_GPL(usb_ep_free_request);
  * arranges to poll once per interval, and the gadget driver usually will
  * have queued some data to transfer at that time.
  *
+ * Note that @req's ->complete() callback must never be called from
+ * within usb_ep_queue() as that can create deadlock situations.
+ *
  * Returns zero, or a negative error code.  Endpoints that are not enabled
  * report errors; errors will also be
  * reported when the usb peripheral is disconnected.