ath9k: fix TX power reporting on AR9003 chips

The current code unconditionally reads the target
power values for all modes from the EEPROM. However
In 'ar9003_hw_set_power_per_rate_table' the regulatory
caps are applied only on a mode specific subset of the
power values.

The reported TX power level is calculated from the
maximum of the power values. Because some of these
values are uncapped in certain cases, the reported
TX power will be wrong.

On the older chipset, we don't have such problems
because only the mode specific subset of the power
levels are retrieved from the EEPROM on those. Do
the same for the AR9003 chips to fix the issue.

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