cfg80211: Add new BSS attribute ht_opmode

Add a new BSS attribute to allow hostapd to set the current HT opmode.
Otherwise drivers won't be able to set up protection for HT rates in
AP mode.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>