mv643xx_eth: fix of_irq_to_resource() error check

of_irq_to_resource() has recently been  fixed to return negative error #'s
along with 0 in case of failure,  however the Marvell MV643xx Ethernet
driver 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: David S. Miller <davem@davemloft.net>