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: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/smsc/smsc911x.c b/drivers/net/ethernet/smsc/smsc911x.c
index da5cc9a..48e2b99 100644
--- a/drivers/net/ethernet/smsc/smsc911x.c
+++ b/drivers/net/ethernet/smsc/smsc911x.c
@@ -2115,7 +2115,7 @@
 	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;
 	}