net/smsc911x: Use NULL instead of integer for pointer

Silences the following sparse warning:
drivers/net/ethernet/smsc/smsc911x.c:2145:30:
warning: Using plain integer as NULL pointer
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/smsc/smsc911x.c

@@ -2115,7 +2115,7 @@ static int smsc911x_init(struct net_device *dev)
 	spin_lock_init(&pdata->dev_lock);
 	spin_lock_init(&pdata->mac_lock);
 
-	if (pdata->ioaddr == 0) {
+	if (pdata->ioaddr == NULL) {
 		SMSC_WARN(pdata, probe, "pdata->ioaddr: 0x00000000");
 		return -ENODEV;
 	}