selftests: mlxsw: Consider VxLAN learning enabled as valid

The test currently expects that a configuration which includes a VxLAN
device with learning enabled to fail.

Previous patches enabled VxLAN learning in mlxsw, so change the test
accordingly.

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