tools/power turbostat: Do not display an error on systems without a cpufreq driver

Running without a cpufreq driver is a valid case so warnings output in
this case should not be to stderr.

Use outf instead of stderr for these warnings.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>