ath9k: call hw initializer directly

ath9k_hw_attach() was going first through some device id verifier,
and then calling some other helper which was doing the real hardware
initialization. Lets just do the devid checks within the real worker
by calling a helper ath9k_hw_devid_supported().

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