ath9k: Allow user to change tx power when asked

We were also changing tx power even when we were not asked to,
this enforces the change only when we are asked nicely. When
not asked we simply try to use the max power, we don't tx power
at all for rate control.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>