ath9k: use ath9k_hw_update_regulatory_maxpower in ath9k_hw_def_set_txpower

We have a helper function for updating the max_power_level
value. Use that and remove the duplicated code.

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