wifi: mt76: connac: add beacon protection support for mt7996

Implement beacon protection feature for mt7996 chipsets, and also do
some cleanup on the set key routine.

Co-developed-by: Rudra Shahi <rudra.shahi@mediatek.com>
Signed-off-by: Rudra Shahi <rudra.shahi@mediatek.com>
Signed-off-by: Allen Ye <allen.ye@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>