iwlwifi: mvm: refactor iwl_mvm_rs_rate_init

Refactor setting of VHT enabled rates in preparation of adding
some more logic there.

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>