pata_of_platform: remove direct dependency on OF_IRQ

CONFIG_OF_IRQ is not available on some platforms and using of_irq_*
breaks the build. Since resources are already populated in the platform
device, get the irq from there instead.

Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>