net/ethernet: ks8851_mll: signedness bug in ks8851_probe()

netdev->irq is unsigned, so it's never less than zero.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Jan Weitzel <j.weitzel@phytec.de>
Signed-off-by: David S. Miller <davem@davemloft.net>