mlxsw: spectrum: Move the ECN-marked packet counter to ethtool

Spectrum-1 and Spectrum-2 do not have a per-TC counter of number of packets
marked by ECN. The value reported currently is the total number of marked
packets. Showing this value at individual TC Qdiscs is misleading.

Move the counter to ethtool instead.

Signed-off-by: Petr Machata <petrm@mellanox.com>
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>