net: calxedaxgmac: fix rx ring handling when OOM

If skb allocation for the rx ring fails repeatedly, we can reach a point
were the ring is empty. In this condition, the driver is out of sync with
the h/w. While this has always been possible, the removal of the skb
recycling seems to have made triggering this problem easier.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/calxeda/xgmac.c

@@ -1141,6 +1141,9 @@ static int xgmac_rx(struct xgmac_priv *priv, int limit)
 		struct sk_buff *skb;
 		int frame_len;
 
+		if (!dma_ring_cnt(priv->rx_head, priv->rx_tail, DMA_RX_RING_SZ))
+			break;
+
 		entry = priv->rx_tail;
 		p = priv->dma_rx + entry;
 		if (desc_get_owner(p))