mt76: mt7915: fix possible deadlock in mt7915_stop

make mac_work per phy instead of per device and fix a possible deadlock
in mt7915_stop since mt7915_mac_work runs holding mt76 mutex

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>