wifi: mt76: mt7996: support per-band LED control

Extend settings of LED registers to support per-band configuration.

Signed-off-by: Jen-Hao Cheng <jen-hao.cheng@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>