[CPUFREQ] Improve Error reporting in powernow-k8

This patch cleans up some error messages in the
powernow-k8 driver and makes them more understandable.

Signed-off-by: Jacob Shin <jacob.shin@amd.com>
Signed-off-by: Mark Langsdorf <mark.langsdorf@amd.com>
Signed-off-by: Dave Jones <davej@redhat.com>