mac80211: (selectively) add HT details in radiotap

Add a flag for the HT format (mixed vs. greenfield)
to allow drivers to report that on receive. Not all
drivers will do that though, so allow drivers to set
which radiotap MCS details they report.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>