wifi: mt76: mt7996: add locking for accessing mapped registers

[ Upstream commit 3687854d

 ]

A race condition was observed when accessing mapped registers, so add
locking to protect against concurrent access.

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