mlxsw: spectrum: Update sch_prio stats to include sch_red related drops

sch_prio as root qdisc should count all the drops its children have. Since
it is possible for it to have sch_red children, it needs to count RED early
drops.

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>