ath9k_htc: Cleanup HTC debugfs

Move the ath9k_htc debugfs under ieee80211 to be inline
with ath9k driver and it also helps to simplify debug code.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Acked-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>