net: bcmgenet: Avoid touching non-existent interrupt

As platform_get_irq() now prints an error when the interrupt does not
exist, we are getting a confusing error message in case the optional
WOL IRQ is not defined:

  bcmgenet fd58000.ethernet: IRQ index 2 not found

Fix this by using the platform_get_irq_optional().

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>