mt76: add mt76x02_phy_set_txpower utility routine

Add mt76x02_phy_set_txpower utility routine in mt76x02_phy.c
in order to be reused in mt76x0 tx power management code.
Moreover move following routines in mt76x02-lib module:
- mt76x02_tx_power_mask
- mt76x02_get_max_rate_power
- mt76x02_limit_rate_power
- mt76x02_add_rate_power_offset

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