net: systemport: Replace platform_get_irq with platform_get_irq_optional

Replace platform_get_irq with platform_get_irq_optional because wol_irq
is optional.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>