mlxsw: spectrum: qdiscs: Support qdisc per tclass

Add the option to set a qdisc per tclass.  Match the qdisc to the tclass by
parent ID. Supported currently for sch_red only.
It allows offloading sch_prio as root qdisc and sch_red as its child.
(However, doing so might corrupt the stats for both parent and child.)

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>