wifi: mt76: mt76x0: fix oob access in mt76x0_phy_get_target_power

After 'commit ba45841c ("wifi: mt76: mt76x02: simplify struct
mt76x02_rate_power")', mt76x02 relies on ht[0-7] rate_power data for
vht mcs{0,7}, while it uses vth[0-1] rate_power for vht mcs {8,9}.
Fix a possible out-of-bound access in mt76x0_phy_get_target_power routine.

Fixes: ba45841c ("wifi: mt76: mt76x02: simplify struct mt76x02_rate_power")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>