mt76: connac: add support for limiting to maximum regulatory Tx power

Consider .max_reg_power in struct ieee80211_channel to limit the
maximum power the wireless device allowed to transmit on the
corresponding channel according to the regulatory domain.

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>