wifi: mt76: change txpower init to per-phy

Use per-phy structure for maximum txpower value initializing, since each
phy may have a different chainmask, which can impact the calculation of
power gain.

Co-developed-by: Allen Ye <allen.ye@mediatek.com>
Signed-off-by: Allen Ye <allen.ye@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>