USB: gadget: f_mtp: dequeue request on error in mtp_read

In mtp_read(), if wait_event_interruptible() returns an error, we need to
remove the request from the EP queue.  Else, on the next call to mtp_read(), we
will attempt to enqueue the request again, potentially corrupting the
queue.  This is what happens with musb_gadget_queue(), which does not check for
duplicate requests.

Based on a similar change to f_adb.c

Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/drivers/usb/gadget/f_mtp.c b/drivers/usb/gadget/f_mtp.c
index 8128b20..92dd89b 100644
--- a/drivers/usb/gadget/f_mtp.c
+++ b/drivers/usb/gadget/f_mtp.c
@@ -504,6 +504,7 @@
 	ret = wait_event_interruptible(dev->read_wq, dev->rx_done);
 	if (ret < 0) {
 		r = ret;
+		usb_ep_dequeue(dev->ep_out, req);
 		goto done;
 	}
 	if (dev->state == STATE_BUSY) {