wifi: mt76: mt7996: implement driver specific get_txpower function

Fixes reporting tx power for vifs that don't have a channel context
assigned. Report the tx power of a phy that is covered by the vif's
radio mask.

Link: https://patch.msgid.link/20250311103646.43346-7-nbd@nbd.name


Signed-off-by: Felix Fietkau <nbd@nbd.name>