net: make ndev->irq signed for error handling

There is a bug in cpsw_probe() where we do:

	ndev->irq = platform_get_irq(pdev, 0);
	if (ndev->irq < 0) {

The problem is that "ndev->irq" is unsigned so the error handling
doesn't work.  I have changed it to a regular int.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>