net: bcmgenet: Add a shutdown callback

Make sure that we completely quiesce the network device, including its
DMA to avoid having it continue to receive packets while there is no
software alive to service those.

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