mt76x0: remove eeprom dependency from mt76x0_set_tx_power_per_rate

In order to unify eeprom parsing between mt76x0 and mt76x2 drivers,
remove eeprom pointer dependency from mt76x0_set_tx_power_per_rate.
Moreover use mt76_rate_power to store power vs rate calibration data.

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