make mac_work per phy instead of per device and fix a possible deadlock
in mt7615_stop since mt7615_mac_work runs holding mt76 mutex

Fixes: fdd2e570 ("mt76: mt7615: add dual-phy support for mac80211 ops")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>