mlxsw: spectrum: Expose per-priority counters via ethtool

Expose per-priority bytes / packets / PFC packets counters via ethtool.

These counters are very useful when debugging QoS functionality and
provide a better insight into the device's forwarding plane.

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