mscc: ocelot: Implement ndo_get_port_parent_id()

Ocelot only supports SWITCHDEV_ATTR_ID_PORT_PARENT_ID as a valid
switchdev attribute getter, convert it to use ndo_get_port_parent_id()
and get rid of the switchdev_ops::switchdev_port_attr_get altogether.

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