net: dsa: add port_fdb_dump function

Not all switch chips support a Get Next operation to iterate on its FDB.
So add a more simple port_fdb_dump function for them.

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