ath9k: Fix MCS rates registration bug for AR9285

AR9285 based devices support only single stream MCS rates.
This patch fixes a bug where dual stream stream rates were
also being registered.

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