iwlwifi: mvm: fix error checking for multi/broadcast sta

The earlier patch called the station add functions but didn't
assign their return value to the ret variable, so that the
checks for it were meaningless. Fix that.

Found by smatch:

.../mac80211.c:2560 iwl_mvm_start_ap_ibss() warn: we tested 'ret' before and it was 'false'
.../mac80211.c:2563 iwl_mvm_start_ap_ibss() warn: we tested 'ret' before and it was 'false'

Fixes: 3a89411cd31c ("iwlwifi: mvm: fix assert 0x2B00 on older FWs")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/mac80211.c

@@ -2132,10 +2132,10 @@ static int iwl_mvm_start_ap_ibss(struct ieee80211_hw *hw,
 		 * Send the bcast station. At this stage the TBTT and DTIM time
 		 * events are added and applied to the scheduler
 		 */
-		iwl_mvm_send_add_bcast_sta(mvm, vif);
+		ret = iwl_mvm_send_add_bcast_sta(mvm, vif);
 		if (ret)
 			goto out_unbind;
-		iwl_mvm_add_mcast_sta(mvm, vif);
+		ret = iwl_mvm_add_mcast_sta(mvm, vif);
 		if (ret) {
 			iwl_mvm_send_rm_bcast_sta(mvm, vif);
 			goto out_unbind;