mt76: mt76x02u: fix possible memory leak in __mt76x02u_mcu_send_msg

Free the skb if mt76u_bulk_msg fails in __mt76x02u_mcu_send_msg routine.

Fixes: 4c89ff2c

 ("mt76: split __mt76u_mcu_send_msg and mt76u_mcu_send_msg routines")
Co-developed-by: Gergo Koteles <soyer@irl.hu>
Signed-off-by: Gergo Koteles <soyer@irl.hu>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>