[PATCH] of_irq_to_resource now returns the virq

Mostly this is to allow for error checking (check the return for NO_IRQ)
Added a check that the resource is non-NULL, too.

Signed-off-by: Andrew Fleming <afleming@freescale.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>