net/mlx5: Properly remove all steering objects

Instead of explicitly cleaning up the well known parts of the steering
tree, we use the generic tree structure to traverse for cleanup.
No functional changes.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>