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>