ath9k: always compile ath_radio_{en,dis}able

ath_radio_{en,dis}able is only compiled if RFKILL is enabled, but it is
required by the 'ath9k_wiphy_select' function.

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