net: dsa: mv88e6xxx: Create API to read a single stat counter

This change contains no functional change. We simply push the hardware
specific stats logic to a function reading a single counter, rather
than the whole set.

This is a preparatory change for the upcoming standard ethtool
statistics support (i.e. "eth-mac", "eth-ctrl" etc.).

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>