mt76: mt76u: add missing release on skb in __mt76x02u_mcu_send_msg

In the implementation of __mt76x02u_mcu_send_msg() the skb is consumed
all execution paths except one. Release skb before returning if
test_bit() fails.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>