net: dsa: mv88e6xxx: rework ATU GetNext

Add and use a fresh documented implementation of the ATU GetNext.

Since it is not necessary to write the MAC address to iterate from, only
do it once directly in the ATU GetNext operation, if the provided ATU
entry structure is not valid. This makes the user code simpler.

Also, there is no need to loop when getting a single ATU entry. So
remove the mv88e6xxx_atu_get helper and add a simpler snippet in
mv88e6xxx_port_db_load_purge to lookup a given MAC address.

The _mv88e6xxx_atu_mac_{read,write} are not used anymore thus remove
them. _mv88e6xxx_atu_data_{read,write} are still used so keep them.

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