net: dsa: mv88e6xxx: rework FDB Get Next operation

Add a low level _mv88e6xxx_atu_getnext function for convenient access to
the hardware, and rework the FDB Get Next operation.

This will ease the future integration with VLAN IDs.

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