wifi: mt76: mt7996: move mt7996_update_beacons under mt76 mutex

wifi: mt76: mt7996: move mt7996_update_beacons under mt76 mutex

Move mt7996_update_beacons routine inside mt76 mutex critical section
in mt7996_mac_reset_work() in order to run mt7996_vif_conf_link() in
mt7996_mcu_add_beacon routine.

Fixes: f30906c55a400 ("wifi: mt76: mt7996: disable beacons when going offchannel")
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-v1-2-259ebf11f654@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>