Documentation: networking: switchdev: Update port parent ID section

Update the section about switchdev drivers having to implement a
switchdev_port_attr_get() function to return
SWITCHDEV_ATTR_ID_PORT_PARENT_ID since that is no longer valid after
commit bccb3025 ("net: Get rid of
SWITCHDEV_ATTR_ID_PORT_PARENT_ID").

Fixes: bccb3025

 ("net: Get rid of SWITCHDEV_ATTR_ID_PORT_PARENT_ID")
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
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>