wifi: mt76: mt7915: fix memory leak in mt7915_mcu_exit

Always purge mcu skb queues in mt7915_mcu_exit routine even if
mt7915_firmware_state fails.

Fixes: e57b7901 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>