Bluetooth: btusb: fix memory leak

If btusb_mtk_submit_wmt_recv_urb returns error, wc should be freed and
then error should be returned to prevent memory leak.

Addresses-Coverity: ("Prevent memory leak")
Fixes: 4cbb375e997d ("Bluetooth: btusb: Fixed too many in-token issue for Mediatek Chip.")
Signed-off-by: Muhammad Usama Anjum <musamaanjum@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>