net: sched: sch_atm: use Qdisc_class_common structure

Even if it is only for classid now, use this common struct a be aligned
with the rest of the classful qdiscs.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>