mlxsw: spectrum: Expose per-tc counters via ethtool

Expose the transmit queue length of each traffic class and the amount of
unicast packets discarded due to insufficient room in the shared buffer.

The first counter allows us to debug user priority to traffic class
mapping, whereas the drop counter is useful when determining shared buffer
configuration.

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>