mlxsw: spectrum_qdisc: Add mlxsw_sp_qdisc_get_class_stats()

Add a wrapper around mlxsw_sp_qdisc_collect_tc_stats() and
mlxsw_sp_qdisc_update_stats() for the simple case of doing both in one go:
mlxsw_sp_qdisc_get_class_stats(). Dispatch to that function from
mlxsw_sp_qdisc_get_red_stats(). This new function will be useful for other
leaf Qdiscs as well.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>