mac80211: disable 40MHz support in case of 20MHz AP

If the AP only advertises support for 20MHz (in the
ht operation ie), disable 40MHz and VHT.

This can improve interoperability with APs that
don't like stations exceeding their own
advertised capabilities.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>