Bluetooth: btusb: Fix memleak in btusb_mtk_submit_wmt_recv_urb

When kmalloc() on buf fails, urb should be freed just like
when kmalloc() on dr fails.

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>