ath9k: use ath_hw for DPRINTF() and debug init/exit

DPRINTF() is used in hw specific related code, as such
ensure we don't rely on the private driver core ath_softc
struct when calling it. Drivers can then implement their
own DPRINTF() as they see fit.

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