ar5523: use container_of

If we want to shrink skb->cb then we'd have to see about
reducing struct ieee80211_tx_info, which gets embedded inside
skb->cb[].

It provides a scratch space to be used by wireless drivers.
ar5523 uses the maximum space available today (40 bytes), but it seems
we don't need this -- data->skb pointer seems to always point back to the
skb whose cb buffer the data structure resides, iow, given a pointer to the
embedded control buffer we can infer the skb address.

Tested-by: Pontus Fuchs <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>

drivers/net/wireless/ath/ar5523/ar5523.c

···
 		ieee80211_stop_queues(hw);
 	}
 
-	data->skb = skb;
-
 	spin_lock_irqsave(&ar->tx_data_list_lock, flags);
 	list_add_tail(&data->list, &ar->tx_queue_pending);
 	spin_unlock_irqrestore(&ar->tx_data_list_lock, flags);
···
 		if (!data)
 			break;
 
-		skb = data->skb;
+		txi = container_of((void *)data, struct ieee80211_tx_info,
+				   driver_data);
 		txqid = 0;
-		txi = IEEE80211_SKB_CB(skb);
+
+		skb = container_of((void *)txi, struct sk_buff, cb);
 		paylen = skb->len;
+
 		urb = usb_alloc_urb(0, GFP_KERNEL);
 		if (!urb) {
 			ar5523_err(ar, "Failed to allocate TX urb\n");

drivers/net/wireless/ath/ar5523/ar5523.h

···
 struct ar5523_tx_data {
 	struct list_head	list;
 	struct ar5523		*ar;
-	struct sk_buff		*skb;
 	struct urb		*urb;
 };