sunvnet: Invoke SET_NETDEV_DEV() to set up the vdev in vnet_new()

`ls /sys/devices/channel-devices/vnet-port-0-0/net' is missing without
this change, and applications like NetworkManager are looking in
sysfs for the information.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>