mt76x0: eeprom: fix VHT mcs{8,9} rate power offset

Fix rate power configuration for VHT mcs 8 and 9 in
mt76x0_get_tx_power_per_rate. Moreover use the rate power
offset used for HT/VHT mcs 6 even for HT/VHT mcs 7

Fixes: b37bbc8c

 ("mt76x0: remove eeprom dependency from mt76x0_set_tx_power_per_rate")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>