mv643xx_eth: fix of_irq_to_resource() error check

of_irq_to_resource() has recently been  fixed to return negative error #'s
along with 0 in case of failure,  however the Marvell MV643xx Ethernet
driver still only regards 0  as invalid IRQ -- fix it up.

Fixes: 7a4228bb ("of: irq: use of_irq_get() in of_irq_to_resource()")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mv643xx_eth.c

@@ -2734,7 +2734,7 @@ static int mv643xx_eth_shared_of_add_port(struct platform_device *pdev,
 	ppd.shared = pdev;
 
 	memset(&res, 0, sizeof(res));
-	if (!of_irq_to_resource(pnp, 0, &res)) {
+	if (of_irq_to_resource(pnp, 0, &res) <= 0) {
 		dev_err(&pdev->dev, "missing interrupt on %s\n", pnp->name);
 		return -EINVAL;
 	}