net: dsa: mv8e6xxx: Fix stub function parameters

mv88e6xxx_g2_atu_stats_get() takes two parameters. Make the stub
function also take two, otherwise we get compile errors.

Fixes: c5f299d5

 ("net: dsa: mv88e6xxx: global1_atu: Add helper for get next")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>