mt76x0: phy: introduce mt76x0_phy_set_txpower routine

Add mt76x0_phy_set_txpower routine in order to configure
base and per rate tx power gain. Moreover take into account
txpower_conf in order to limit the tx power gain according to
the bound provided by mac80211

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>