selftests: mlxsw: Adjust test to recent changes

mlxsw does not support VXLAN devices with a physical device attached and
vetoes such configurations upon enslavement to an offloaded bridge.

Commit 0ce1822c ("vxlan: add adjacent link to limit depth level")
changed the VXLAN device to be an upper of the physical device which
causes mlxsw to veto the creation of the VXLAN device with "Unknown
upper device type".

This is OK as this configuration is not supported, but it prevents us
from testing bad flows involving the enslavement of VXLAN devices with a
physical device to a bridge, regardless if the physical device is an
mlxsw netdev or not.

Adjust the test to use a dummy device as a physical device instead of a
mlxsw netdev.

Fixes: 0ce1822c

 ("vxlan: add adjacent link to limit depth level")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>