selftests: forwarding: vxlan_bridge_1d: use more proper tos value

0x11 and 0x12 set the ECN bits based on RFC2474, it would be better to avoid
that. 0x14 and 0x18 would be better and works as well.

Reported-by: Petr Machata <petrm@mellanox.com>
Fixes: 4e867c9a ("selftests: forwarding: vxlan_bridge_1d: fix tos value")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>