net: bcmgenet: Turn on offloads by default

We can turn on the RX/TX checksum offloads and the scatter/gather
features by default and make sure that those are properly reflected
back to e.g: stacked devices such as VLAN.

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