mlxsw: spectrum: Dump LAG FDB records only once

LAG FDB records can only point to LAG devices or VLAN devices configured
on top of them. Therefore, when dumping the FDB we shouldn't associate
these records with the underlying physical ports.

Fixes: 8a1ab5d7 ("mlxsw: spectrum: Implement FDB add/remove/dump for LAG")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>