mt7601u: add missing release on skb in mt7601u_mcu_msg_send

In the implementation of mt7601u_mcu_msg_send(), skb is supposed to be
consumed on all execution paths. Release skb before returning if
test_bit() fails.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200718052630.11032-1-navid.emamdoost@gmail.com