mlxsw: spectrum: qdiscs: Make the clean stats function to be for RED only

Improve readability by changing the clean stats function to handle only
RED. Qdiscs that will be offloaded in the future will have a clean stats
function of their own.

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>