sis900: Fix mem leak in sis900_rx error path

Fix memory leak in error path of sis900_rx(). If we don't do this we'll
leak the skb we dev_alloc_skb()'ed just a few lines above when the
variable goes out of scope.
Signed-off-by: NJesper Juhl <jj@chaosbits.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sis900.c

@@ -1777,6 +1777,7 @@ static int sis900_rx(struct net_device *net_dev)
 					      "cur_rx:%4.4d, dirty_rx:%4.4d\n",
 					      net_dev->name, sis_priv->cur_rx,
 					      sis_priv->dirty_rx);
+				dev_kfree_skb(skb);
 				break;
 			}