pata-rb532-cf: platform_get_irq() fix ignored failure

platform_get_irq() can return -ENXIO, but since 'irq' is an
unsigned int, it does not show when the IRQ resource wasn't found.
Make irq an int so that we can use a single variable to test the
platform_get_irq() return value.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>