mt76: mt7615: add missing settings for simultaneous dual-band support

MT7615 supports dual-wiphy which means that it can run on 2.4G and 5GHz
channels simultaneously, and driver should configure each band.

Add missing register settings, and refine band related definitions to
avoid duplicate codes.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>