net: mv643xx_eth: fix build failure

The build of sparc allmodconfig fails with the error:
"of_irq_to_resource" [drivers/net/ethernet/marvell/mv643xx_eth.ko]
	undefined!

of_irq_to_resource() is defined when CONFIG_OF_IRQ is defined. And also
CONFIG_OF_IRQ can only be defined if CONFIG_IRQ is defined. So we can
safely use #if defined(CONFIG_OF_IRQ) in the code.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>