ath9k: get rid of unnecessary setpower calls

We are using setpower routines regardless of the current power mode.
Don't bother the hardware, if it is not necessary.

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