ath9k: reuse ar9003_hw_tx_power_regwrite for tx99 setup

The same functionality as ar9003_hw_tx_power_regwrite is hardcoded in
ar9003_hw_tx99_set_txpower. Just reuse the existing ar9003_hw_tx_power_regwrite
for TX99 setup too.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>