wifi: mt76: mt7996: grab mt76 mutex in mt7996_mac_sta_event()

wifi: mt76: mt7996: grab mt76 mutex in mt7996_mac_sta_event()

[ Upstream commit 5a4bcba26e9fbea87507a81ad891e70bb525014f ]

Grab mt76 mutex in mt7996_mac_sta_event routine in order to rely on
mt76_dereference() utility macro.

Fixes: ecd72f9695e7e ("wifi: mt76: mt7996: Support MLO in mt7996_mac_sta_event()")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Ben Greear <greearb@candelatech.com>
Link: https://patch.msgid.link/20251114-mt76-fix-missing-mtx-v1-1-259ebf11f654@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>