Fix potential memory leak in mcu message handler on error condition.
Fixes: eb99cc95c3b6 ("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>