mlxsw: Implement ndo_get_port_parent_id()

mlxsw implements SWITCHDEV_ATTR_ID_PORT_PARENT_ID and we want to get rid
of switchdev_ops eventually, ease that migration by implementing a
ndo_get_port_parent_id() function which returns what
switchdev_port_attr_get() would do.

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