devlink: remove unused priv_size

Remove unused and useless priv_size member from struct devlink_ops.

Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>