net: dsa: mv88e6xxx: Fix statistics on mv88e6161

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: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>