net: dsa: mv88e6xxx: make switchdev DB ops generic

The MDB support for the mv88e6xxx driver will be very similar to the FDB
support, since it consists of loading/purging/dumping address to/from
the Address Translation Unit (ATU).

Prepare the support for MDB by making the FDB code accessing the ATU
generic. The FDB operations now provide access to the unicast addresses
while the MDB operations will provide access to the multicast addresses.

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