net: dsa: mv88e6xxx: Limit histogram counters to ingress traffic

Chips in this family only have one set of histogram counters, which
can be used to count ingressing and/or egressing traffic. mv88e6xxx
has, up until this point, kept the hardware default of counting both
directions.

In the mean time, standard counter group support has been added to
ethtool. Via that interface, drivers may report ingress-only and
egress-only histograms separately - but not combined.

In order for mv88e6xxx to maximize amount of diagnostic information
that can be exported via standard interfaces, we opt to limit the
histogram counters to ingress traffic only. Which will allow us to
export them via the standard "rmon" group in an upcoming commit.

The reason for choosing ingress-only over egress-only, is to be
compatible with RFC2819 (RMON MIB).

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>