mt76: mt7921: not support beacon offload disable command

Beacon disable flow would be handled in bss stop handler automatically.
Force return -EOPNOTSUPP in disable case.

Fixes: 116c6960

 ("mt76: mt7921: Add AP mode support")
Reviewed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>