wifi: mt76: sdio: move mcu queue size check inside critical section

Even if it is not a real issue at the moment since concurrent access to
mcu message queue is protected by mcu mutex, make the code more robust
and move mcu queue free space check inside queue spinlock critical section.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>