mac80211: refactor ieee80211_set_qos_hdr

Return early if not a QoS Data frame.
Give proper documentation.
Signed-off-by: Marco Porsch <marco.porsch@etit.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/wme.c

@@ -160,17 +160,24 @@ u16 ieee80211_select_queue(struct ieee80211_sub_if_data *sdata,
 	return ieee80211_downgrade_queue(sdata, skb);
 }
 
+/**
+ * ieee80211_set_qos_hdr - Fill in the QoS header if there is one.
+ *
+ * @sdata: local subif
+ * @skb: packet to be updated
+ */
 void ieee80211_set_qos_hdr(struct ieee80211_sub_if_data *sdata,
 			   struct sk_buff *skb)
 {
 	struct ieee80211_hdr *hdr = (void *)skb->data;
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
-
-	/* Fill in the QoS header if there is one. */
-	if (ieee80211_is_data_qos(hdr->frame_control)) {
-		u8 *p = ieee80211_get_qos_ctl(hdr);
+	u8 *p;
 	u8 ack_policy, tid;
 
+	if (!ieee80211_is_data_qos(hdr->frame_control))
+		return;
+
+	p = ieee80211_get_qos_ctl(hdr);
 	tid = skb->priority & IEEE80211_QOS_CTL_TAG1D_MASK;
 
 	/* preserve EOSP bit */
@@ -186,5 +193,4 @@ void ieee80211_set_qos_hdr(struct ieee80211_sub_if_data *sdata,
 	*p++ = ack_policy | tid;
 	*p = ieee80211_vif_is_mesh(&sdata->vif) ?
 		(IEEE80211_QOS_CTL_MESH_CONTROL_PRESENT >> 8) : 0;
-	}
 }