mac80211: add STBC flag for radiotap

Some chips can tell us if received frame was
encoded with STBC or not. To make this information available
in user space we can use updated radiotap specification:
http://www.radiotap.org/defined-fields/MCS



This patch will set number of STBC encoded spatial streams (Nss).
The HAVE_STBC flag should be provided by driver.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>