mt76: mt7921: introduce mt7921_mcu_sta_add routine

mt7921_mcu_sta_add will be used to add and remove wtbl entries.
Create broadcast wtbl entry after AP association

Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>