selftests: mlxsw: extack: Test creation of multiple VLAN-aware bridges

The driver supports a single VLAN-aware bridge. Test that the
enslavement of a port to the second VLAN-aware bridge fails with an
extack.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>