iwlwifi: introduce iwl_sta_id_or_broadcast

There are now five places where we need to
look up the station ID, but the sta pointer
may be NULL due to mac80211 passing that to
indicate a certain special state.

Replace all these by a new inline function,
called iwl_sta_id_or_broadcast(), and add
documentation about when to use it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-sta.c b/drivers/net/wireless/iwlwifi/iwl-sta.c
index 85ed235..2eafba6 100644
--- a/drivers/net/wireless/iwlwifi/iwl-sta.c
+++ b/drivers/net/wireless/iwlwifi/iwl-sta.c
@@ -972,24 +972,16 @@
 	unsigned long flags;
 	int i;
 
-	if (sta) {
-		sta_id = iwl_sta_id(sta);
-
-		if (sta_id == IWL_INVALID_STATION) {
-			IWL_DEBUG_MAC80211(priv, "leave - %pM not initialised.\n",
-					   sta->addr);
-			return;
-		}
-	} else
-		sta_id = priv->hw_params.bcast_sta_id;
-
-
 	if (iwl_scan_cancel(priv)) {
 		/* cancel scan failed, just live w/ bad key and rely
 		   briefly on SW decryption */
 		return;
 	}
 
+	sta_id = iwl_sta_id_or_broadcast(priv, sta);
+	if (sta_id == IWL_INVALID_STATION)
+		return;
+
 	spin_lock_irqsave(&priv->sta_lock, flags);
 
 	priv->stations[sta_id].sta.key.tkip_rx_tsc_byte2 = (u8) iv32;