ath5k: Skip powertable setting when we are on the same channel

 * Only set power table if we are changing channel/mode
 there is no need to recalculate and reset the power table
 all the time.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>