mt76: mt7921: fix OMAC idx usage

OMAC idx have to be same with BSS idx according to firmware usage.

Fixes: e0f9fdda ("mt76: mt7921: add ieee80211_ops")
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: YN Chen <yn.chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>