ath9k: Fix multi-VIF BSS handling

mac80211 provides short preamble information and ERP protection
information on a per-BSS basis, which can be used. Remove flags
stored in the driver, which was incorrect since they were being used
in a global manner.

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