mlxsw: spectrum_qdisc: Extract a per-TC stat function

Extract from mlxsw_sp_qdisc_get_prio_stats() two new functions:
mlxsw_sp_qdisc_collect_tc_stats() to accumulate stats for that one TC only,
and mlxsw_sp_qdisc_update_stats() that makes the stats relative to base
values stored earlier. Use them from mlxsw_sp_qdisc_get_red_stats().

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>