net: bcmgenet: handle GENET_POWER_WOL_MAGIC

Update bcmgenet_power_{up,down} to handle the case where the adapter has
been suspend respectively resumed from Wake-on-LAN using MagicPackets.

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