mt76: mt7915: rework the flow of txpower setting

Clean up the flow of per-rate txpower limit setting to get rid of
duplicate work since it has already been handled by firmware, and set
proper max_power based on different channels and regdomains.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>