wifi: mt76: mt7996: add txpower setting support

Add support for setting txpower from upper layer and configuring per-rate
txpower limit table.

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