iwlwifi: mvm: don't configure mimo rates if nss is limited to 1

Remote peer can publish a different number of supported nss via the
operating mode notification IE or action frame. If it limits to 1
then we don't want mimo rates configured in the rate table.

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