mac80211: disable HT advertising unless AP supports it

If the AP doesn't support HT, or more importantly if
it does but we have to disable it because its IEs are
broken, don't advertise HT support in our association
request. Otherwise, we configure our channel to be a
20 MHz non-HT channel but the AP might still think we
support HT, or even 40 MHz.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>