ath9k: Fill remove_sta_debugfs() callback

Remove the rate control statistics debugfs file properly
via remove_sta_debugfs(). Also, check for both MAC80211_DEBUGFS
and ATH9K_DEBUGFS config options.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>