Despite what the datesheet says, the silicon implements the older way
of snapshoting the statistics. Change the op.

Reported-by: Chris.Healy@zii.aero
Tested-by: Chris.Healy@zii.aero
Fixes: 0ac64c39 ("net: dsa: mv88e6xxx: mv88e6161 uses mv88e6320 stats snapshot")
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>