mt76: mt7615: fix getting maximum tx power from eeprom

On top of the EEPROM target power, each rate can also has a power offset.
On many devices, this power offset is used to boost the tx power of lower
rates. Take this into account when parsing rate power.
The assumption here is, that the first rate (OFDM 6M or CCK 1M) has the
highest tx power

Signed-off-by: Felix Fietkau <nbd@nbd.name>