gianfar: fix handle errors returned by platform_get_irq*()

platform_get_irq*() returns on -ENXIO when the resource cannot be
found, but this remains unnoticed if stored in an unsigned.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>