mt76: mt7615: take into account extPA when configuring tx power

When TSSI calibration is disabled (which it means the device has been
equipped with an external power amplifier) we need to refer to
different eeprom fields in order to properly configure tx power

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>