wifi: mt76: mt7921: move mac shared code in mt792x-lib module

Reduce duplicated code moving mac shared code in mt792x-lib module.

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