mt76: mt7615: fix possible memory leak in mt7615_mcu_wtbl_sta_add

Free the second mcu skb if __mt76_mcu_skb_send_msg() fails to transmit
the first one in mt7615_mcu_wtbl_sta_add().

Fixes: 99c457d9

 ("mt76: mt7615: move mt7615_mcu_set_bmc to mt7615_mcu_ops")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>