net/sched: taprio: report class offload stats per TXQ, not per TC

The taprio Qdisc creates child classes per netdev TX queue, but
taprio_dump_class_stats() currently reports offload statistics per
traffic class. Traffic classes are groups of TXQs sharing the same
dequeue priority, so this is incorrect and we shouldn't be bundling up
the TXQ stats when reporting them, as we currently do in enetc.

Modify the API from taprio to drivers such that they report TXQ offload
stats and not TC offload stats.

There is no change in the UAPI or in the global Qdisc stats.

Fixes: 6c1adb65 ("net/sched: taprio: add netlink reporting for offload statistics counters")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>