mt76: mt7615: rework locking scheme for mt7615_set_channel

As already done for mt7603 driver, move mt76.mutex lock inside
mt7615_set_channel since we need to grab mt76.mutex in mt7615_mac_work.
This is a preliminary patch to add Smart Carrier Sense (SCS) support

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