ath9k: Fix ATH9K_HW_CAP_HT usage

There are a few places where the station's HT capabilities
should be checked instead of ATH9K_HW_CAP_HT, which is a global
feature for the driver. Fix this.

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