wifi: mt76: mt7915: add locking for accessing mapped registers

[ Upstream commit 0937f95a

 ]

Sicne the mapping is global, mapped register access needs to be protected
against concurrent access, otherwise a race condition might cause the reads
or writes to go towards the wrong register

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>