net/mlx5: Don't maintain a case of del_sw_func being null

Add del_sw_func cb for root ns. Now there is no need to
maintain a case of del_sw_func being null when freeing the node.

Fixes: 2cc43b49

 ("net/mlx5_core: Managing root flow table")
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>