selftests: mlxsw: Test egress sampling limitation on Spectrum-1 only

Make sure egress sampling configuration only fails on Spectrum-1, given
that mlxsw now supports it on Spectrum-{2,3}.

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