net: bcmgenet: propagate errors from bcmgenet_power_down

If bcmgenet_power_down() fails, we would want to propagate a return
value from bcmgenet_wol_power_down_cfg() to know about this.

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