wifi: mt76: mt7996: disable beacons when going offchannel

Avoid leaking beacons on unrelated channels during scanning/roc

Fixes: c56d6ede ("wifi: mt76: mt7996: use emulated hardware scan support")
Reported-by: Chad Monroe <chad.monroe@adtran.com>
Link: https://patch.msgid.link/20250813121106.81559-1-nbd@nbd.name


Signed-off-by: Felix Fietkau <nbd@nbd.name>