ath9k: get correct tx gain type in ath9k_hw_4k_get_eeprom

The base_eep_header_4k structure contains information that the
device supports high power tx gain table or not. However the
ath9k_hw_4k_get_eeprom function does not return that value when
it is called with EEP_TXGAIN_TYPE. This leads to that the tx gain
initialization will use the init values from the original tx gain
table even if the device inidicates that the high power table
should be used.

Changes-licensed-under: ISC
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Shu Hwa Shen <shensh@zcomm.com.cn>
Signed-off-by: John W. Linville <linville@tuxdriver.com>