Fix potential memory leak in mcu message handler on error condition.

Fixes: eb99cc95 ("mt76: mt7615: introduce mt7663u support")
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>