mlxsw: spectrum_qdisc: Introduce per-TC ECN counters

The Qdisc code in mlxsw used to report a number of packets ECN-marked on a
port. Because reporting a per-port value as a per-TC value was misleading,
this was removed in commit 8a29581e ("mlxsw: spectrum: Move the
ECN-marked packet counter to ethtool").

On Spectrum-3, a per-TC number of ECN-marked packets is available in per-TC
congestion counter group. Add a new array for the ECN counter, fetch the
values from the per-TC congestion group, and pick the value indicated by
tclass_num as appropriate.

On Spectrum-1 and Spectrum-2, this per-TC value is not available, and
zeroes will be reported, as they currently are.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>