net/mlx5: Fix autogroups groups num not decreasing

Autogroups groups num is increased when creating a new flow group,
but is never decreased.

Now decreasing it when deleting a flow group.

Fixes: f0d22d18 ('net/mlx5_core: Introduce flow steering autogrouped flow table')
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>