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-agn-tx.c b/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
index c402bfc..18b1546 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
@@ -567,10 +567,7 @@
 	hdr_len = ieee80211_hdrlen(fc);
 
 	/* Find index into station table for destination station */
-	if (!info->control.sta)
-		sta_id = priv->hw_params.bcast_sta_id;
-	else
-		sta_id = iwl_sta_id(info->control.sta);
+	sta_id = iwl_sta_id_or_broadcast(priv, info->control.sta);
 	if (sta_id == IWL_INVALID_STATION) {
 		IWL_DEBUG_DROP(priv, "Dropping - INVALID STATION: %pM\n",
 			       hdr->addr1);