net: devlink: extend port attrs for switch ID

Extend devlink_port_attrs_set() to pass switch ID for ports which are
part of switch and store it in port attrs. For other ports, this is
NULL.

Note that this allows the driver to group devlink ports into one or more
switches according to the actual topology.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>