ath9k: Fix bug in handling single stream stations

AP mode currently sets up the dual stream capability
for all stations. This patch fixes it by checking if the
associated station supports dual stream MCS rates (8-15).
We would disregard any MCS rates above 15, since Atheros
HW supports only 0..15 rates currently, and can't receive
at rates > 15 anyway.

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