mlxsw: spectrum: qdiscs: Add an ops struct

Qdisc struct have the Qdisc_class_ops struct.
This patch introduces the similar ops struct for the mlxsw_sp_qdisc_ops
struct. It allows better readability as well as code reusability for the
common parts of some functions like destroy.
The first operations to be added are the statistics getters.

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>