iwlwifi: mvm: rs: enable forcing single stream Tx decision

In certain testing scenarios we'd like to force a decision
between STBC/BFER/SISO. In the normal scenario this decision
is done by the FW. Enable this option vis debugfs.

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>