ath9k: fix access to freed data on unload

Calling ath_bus_cleanup() after ieee80211_free_hw() resulted in access
to common->bus_ops, which is already freed as part of the device data.

Remove the cleanup field in struct ath_bus_ops, as it was never used
properly.  Remove ath_bus_cleanup().  Merge cleanup functions in place
of the ath_bus_cleanup() calls.  Take care not to use any device data
after ieee80211_free_hw().

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>