net: WIZnet drivers: fix possible NULL dereference

This fixes possible null dereference in probe() function: when both
.mac_addr and .link_gpio are unknown, dev.platform_data may be NULL

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mike Sinkovsky <msink@permonline.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/wiznet/w5100.c

@@ -682,7 +682,7 @@ static int __devinit w5100_hw_probe(struct platform_device *pdev)
 		return ret;
 	priv->irq = irq;
 
-	priv->link_gpio = data->link_gpio;
+	priv->link_gpio = data ? data->link_gpio : -EINVAL;
 	if (gpio_is_valid(priv->link_gpio)) {
 		char *link_name = devm_kzalloc(&pdev->dev, 16, GFP_KERNEL);
 		if (!link_name)