mlxsw: spectrum: qdiscs: Remove qdisc before setting a new one

If a qdisc is being replaced by another qdisc of the same type, it can
simply override over its configuration.
However, if it replaces a qdisc of another type, it needs to be removed
before setting the new qdisc.

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>