selftests: mlxsw: vxlan: Adjust test to recent changes

After recent changes, the VXLAN tunnel will be offloaded regardless if
any local ports are member in the FID or not. Adjust the test to make
sure the tunnel is offloaded in this case.

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