mlxsw: spectrum: qdiscs: Support stats for PRIO qdisc

Support basic stats for PRIO qdisc, which includes tx packets and bytes
count, drops count and backlog size. The rest of the stats are irrelevant
for this qdisc offload.
Since backlog is not only incremental but reflecting momentary value, in
case of a qdisc that stops being offloaded but is not destroyed, backlog
value needs to be updated about the un-offloading.
For that reason an unoffload function is being added to the ops struct.

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>