of: irq: fix of_irq_to_resource() error check

of_irq_to_resource() has recently been fixed to return negative error #'s
along with 0, however of_irq_to_resource_table() still only regards 0 as
invalid IRQ -- fix it up.

Fixes: 7a4228bb ("of: irq: use of_irq_get() in of_irq_to_resource()")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <robh@kernel.org>