mt76: mt7615: fix and rework tx power handling

Setting the tx power by manipulating EEPROM may in some cases not be enough,
since it only covers the base target power and not per-rate offsets.
In other cases, it could limit tx power of rates too much, possibly reducing
throughput or range.
Use firmware support for applying per-rate limit and power offsets for
different values of Nss.

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