net/mlx5: Fix cleaning unmanaged flow tables

Unmanaged flow tables doesn't have a parent and tree_put_node()
assume there is always a parent if cleaning is needed. fix that.

Fixes: 5281a0c9

 ("net/mlx5: fs_core: Introduce unmanaged flow tables")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>