mlxsw: spectrum_qdisc: Validate qdisc topology

A following patch will enable offloading qdiscs that are deeper than
directly under root qdisc. Currently the topology validation consists of
demanding a root qdisc position for ETS and PRIO. Since RED and TBF are
considered classless, this is enough. In order to prevent some nonsensical
combinations when RED and TBF become classful, introduce a more general
topology validator.

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