bcmgenet: add WOL IRQ check

The driver neglects to check the result of platform_get_irq_optional()'s
call and blithely passes the negative error codes to devm_request_irq()
(which takes *unsigned* IRQ #), causing it to fail with -EINVAL.
Stop calling devm_request_irq() with the invalid IRQ #s.

Fixes: 8562056f

 ("net: bcmgenet: request Wake-on-LAN interrupt")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>