dsa: implement ndo_get_devlink_port

In order for devlink compat functions to work, implement
ndo_get_devlink_port. Legacy slaves does not have devlink port instances
created for themselves.

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