mlxsw: spectrum: Don't take multiple references on a FID

In commit 14d39461 ("mlxsw: spectrum: Use per-FID struct for the
VLAN-aware bridge") I added a per-FID struct, which member ports can
take a reference on upon VLAN membership configuration.

However, sometimes only the VLAN flags (e.g. egress untagged) are
toggled without changing the VLAN membership. In these cases we
shouldn't take another reference on the FID.

Fixes: 14d39461 ("mlxsw: spectrum: Use per-FID struct for the VLAN-aware bridge")
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>