iwlwifi: mvm: rs: enable MCS9 for Tx

After fixes to the rs algorithm reenable MCS9.

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>