[CPUFREQ] powernow-k8: Only print error message once, not per core.

This is the typical message you get if you plug in a CPU
which is newer than your BIOS. It's annoying seeing this
message for each core.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Dave Jones <davej@redhat.com>