net: dsa: mv88e6xxx: handle multiple ports in ATU

An address can be loaded in the ATU with multiple ports, for instance
when adding multiple ports to a Multicast group with "bridge mdb".

The current code doesn't allow that. Add an helper to get a single entry
from the ATU, then set or clear the requested port, before loading the
entry back in the ATU.

Note that the required _mv88e6xxx_atu_getnext function is defined below
mv88e6xxx_port_db_load_purge, so forward-declare it for the moment. The
ATU code will be isolated in future patches.

Fixes: 83dabd1f

 ("net: dsa: mv88e6xxx: make switchdev DB ops generic")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>