ath9k: Fix rate control debugging

Update the rate statistics only when debugfs has been enabled
in ath9k and mac80211 and move the stat() functions under proper
conditionals.

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