ath9k: add a helper to clean the core driver upon module unload

The core driver needs to be stopped and then as a last step the
hardware needs to be stopped and its structure free'd. We do this
by moving the core driver cleanup to a new helper ath_clean_core()
and have ath_cleanup() call it. Only as a last step does
ath_cleanup() now free the hw.

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