net: bcmgenet: request Wake-on-LAN interrupt

Attempt to the request the Wake-on-LAN interrupt bit, and if successful,
advertise wakeup capability instead of doing this unconditionnally.

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