ath9k_hw: use the configured power limit for AR9003

Since the new AR9003 EEPROM code does tune the card for the configured
tx power level, we need to fill in the correct power limits in the TPC
part of the DMA descriptor.

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