mt76: mt7615: hold mt76 lock queueing wd in mt7615_queue_key_update

wq queue is always updated holding mt76 spinlock. Grab mt76 lock in
mt7615_queue_key_update() before putting a new element at the end of the
queue.

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