mt76: move mac_work in mt76_core module

Move mac_work delayed_work and mac_work_count in mt76_phy since it is
used by all drivers. This is a preliminary patch to create a common mcu
library used by mt7615 and mt7921 drivers

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>