wifi: mt76: mt7615: add mt7615_mutex_acquire/release in mt7615_sta_set_decap_offload

Similar to mt7921 driver, introduce mt7615_mutex_acquire/release in
mt7615_sta_set_decap_offload in order to avoid sending mcu commands
while the device is in low-power state.

Fixes: d4b98c63

 ("mt76: mt7615: add support for rx decapsulation offload")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>