ath9k_hw: fix PA predistortion training power selection

The EEPROM contains scale factors for the tx power, which define
the range of allowable difference between target power and training
power. If the difference is too big, PA predistortion cannot be used.
For 2.4 GHz there is only one scale factor, for 5 GHz there are
three, depending on the specific frequency range.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>