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>