Fix this sparse warnings:
  drivers/net/r6040.c:487:31: warning: Using plain integer as NULL pointer
  drivers/net/r6040.c:492:31: warning: Using plain integer as NULL pointer
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
        /* Free Descriptor memory */
        if (lp->rx_ring) {
                pci_free_consistent(pdev, RX_DESC_SIZE, lp->rx_ring, lp->rx_ring_dma);
-               lp->rx_ring = 0;
+               lp->rx_ring = NULL;
        }
 
        if (lp->tx_ring) {
                pci_free_consistent(pdev, TX_DESC_SIZE, lp->tx_ring, lp->tx_ring_dma);
-               lp->tx_ring = 0;
+               lp->tx_ring = NULL;
        }
 
        return 0;