p54: tx refused but queue active

In the mainline kernel, p54usb will fail because the TX queue length can
become < 0. This problem has been reported as Bugzilla #13725. The failure
is expressed by the following message in the logs:

WARNING: at net/mac80211/tx.c:1325 ieee80211_tx+0x23c/0x298 [mac80211]()
Hardware name: HP Pavilion dv2700 Notebook PC
tx refused but queue active

This problem has been recently observed in the wireless-testing tree, where
a full solution is being tested. That fix is too invasive for 2.6.31-rcX,
but the simple change supplied here will prevent the failure.
Signed-off-by: NLarry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/p54common.c

@@ -912,13 +912,14 @@ static void p54_rx_frame_sent(struct ieee80211_hw *dev, struct sk_buff *skb)
 		}
 
 		__skb_unlink(entry, &priv->tx_queue);
-		spin_unlock_irqrestore(&priv->tx_queue.lock, flags);
 
 		frame_len = entry->len;
 		entry_hdr = (struct p54_hdr *) entry->data;
 		entry_data = (struct p54_tx_data *) entry_hdr->data;
-		priv->tx_stats[entry_data->hw_queue].len--;
+		if (priv->tx_stats[entry_data->hw_queue].len)
+			priv->tx_stats[entry_data->hw_queue].len--;
 		priv->stats.dot11ACKFailureCount += payload->tries - 1;
+		spin_unlock_irqrestore(&priv->tx_queue.lock, flags);
 
 		/*
 		 * Frames in P54_QUEUE_FWSCAN and P54_QUEUE_BEACON are