p54pci: rx frame length check

A long time ago, a user reported several crashes due to data corruptions which are likely the result of a not-100%-supported, or faulty? PCI bridge.
( http://patchwork.kernel.org/patch/53004/ )

This patch fixes entry #1.
"1. p54p_check_rx_ring - skb_over_panic: Under a ping flood or just left running for a bit would panic with a skb_over_panic." As described in the mail: The invalid frame length causes skb_put to bailout and trigger a crash.

---
Note:
Simply dropping the frame is problematic, because if its content contains a tx feedback we would lose some portion of the device memory space.... And the driver/mac80211 should handle all other invalid data.
---

index a227c3d..48cae48 100644
@@ -205,6 +205,14 @@ static void p54p_check_rx_ring(struct ieee80211_hw *dev, u32 *index,
 			i %= ring_limit;
 			continue;
 		}
+
+		if (unlikely(len > priv->common.rx_mtu)) {
+			if (net_ratelimit())
+				dev_err(&priv->pdev->dev, "rx'd frame size "
+					"exceeds length threshold.\n");
+
+			len = priv->common.rx_mtu;
+		}
 		skb_put(skb, len);
 
 		if (p54_rx(dev, skb)) {