arcnet: fix signed bug in probe function

probe_irq_off() returns the first irq found or if two irqs are found
then it returns the negative of the first irq found.  We can cast
dev->irq to an int so that the test for negative values works.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>