ath9k_htc: Cleanup rate initialization

This patch removes a large chunk of code dealing
with rate management within the driver and simplifying
things by removing the hacky method of calculating
HT changes. A subsequent patch would fix this by
just using BSS_CHANGED_HT from mac80211.

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