mt76: move mt76_rate_power in mt76_dev

Move mt76_rate_power data structure from mt76x2_dev to mt76_dev in order
to share it with mt76x0 driver. Moreover move txpower_conf and
txpower_cur in mt76_dev

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