wifi: mt76: mt7996: fix memory leak in mt7996_mcu_exit

Always purge mcu skb queues in mt7996_mcu_exit routine even if
mt7996_firmware_state fails.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>