selftests: mlxsw: Adjust test regarding VID 1

Previous patches made it possible to create VLAN devices with VID 1 over
mlxsw ports. Adjust the test to verify such an operation succeeds.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>