bgmac: Bind net_device with backing device structure

In preparation for allowing different helpers to be utilized against
network devices created by the bgmac driver, make sure that we bind the
net_device with core->dev.

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