mt76: move chainmask in mt76_phy

Move chainmask from driver phy to 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>