iwlwifi: mvm: use a new API for enabling STBC

The new API tells the FW that it's allowed to use STBC
but the FW will decide on its own whether to use STBC
or SISO (and in the future Beamformer).
Keep support for the old API which sets STBC explicitly
in the rates in the LQ table while we still support old
FW revisions.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>