net: dsa: mv88e6xxx: global1_atu: Add helper for get next

When retrieving the ATU statistics, and ATU get next has to be
performed to trigger the ATU to collect the statistics. Export a
helper from global1_atu to perform this.

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