switchdev: Replace port obj add/del SDO with a notification

Drop switchdev_ops.switchdev_port_obj_add and _del. Drop the uses of
this field from all clients, which were migrated to use switchdev
notification in the previous patches.

Add a new function switchdev_port_obj_notify() that sends the switchdev
notifications SWITCHDEV_PORT_OBJ_ADD and _DEL.

Update switchdev_port_obj_del_now() to dispatch to this new function.
Drop __switchdev_port_obj_add() and update switchdev_port_obj_add()
likewise.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>