ath9k: optimize power level initialization for CTL_[25]GHT20

The first part of the power array is initialized in a loop
and the last two values are initialized separately. Extend
the loop to cover the last two items, and remove the separate
initialization.

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