ath9k: make tx power config changes take effect immediatley

Users wishing to tweak tx power want it to happen immediately,
try to respect that. This was tested by Lorenzo by measuring the
received signal strength from an AP with ath9k and the patch.

Changing the tx power on the AP produced these results:

1) iwconfig wlan0 txpower 20 ---> Rx power -37dbm
2) iwconfig wlan0 txpower 15 ---> Rx power -41dbm
3) iwconfig wlan0 txpower 10 ---> Rx power -45dbm
4) iwconfig wlan0 txpower 5 ---> Rx power -51dbm
5) iwconfig wlan0 txpower 0 ---> Rx power -37dbm

The result with 0 is an anomoly and would need to be
addressed through a separate patch.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Tested-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>