net: dsa: add support for switchdev FDB objects

Remove the fdb_{add,del,getnext} function pointer in favor of new
port_fdb_{add,del,getnext}.

Implement the switchdev_port_obj_{add,del,dump} functions in DSA to
support the SWITCHDEV_OBJ_PORT_FDB objects.

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