net: dsa: mv88e6xxx: Give each hw stat an ID

With the upcoming standard counter group support, we are no longer
reading out the whole set of counters, but rather mapping a subset to
the requested group.

Therefore, create an enum with an ID for each stat, such that
mv88e6xxx_hw_stats[] can be subscripted with a human-readable ID
corresponding to the counter's name.

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