net: dsa: push prepare phase in port_fdb_add

Now that the prepare phase is pushed down to the DSA drivers, propagate
it to the port_fdb_add function.

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