ath5k: use ctl settings based on current regdomain

Update ath5k to use the ctl settings for tx power based on current
regulatory domain.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>