net: dsa: mv88e6xxx: add a stats setup function

Now that the Global 1 specific setup function only setup the statistics
unit, kill it in favor of a mv88e6xxx_stats_setup function.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>