[CPUFREQ] powernow-k8: Fix misleading variable naming

rdmsr() takes the lower 32 bits as a second argument and the high 32 as
a third. Fix the names accordingly since they were swapped.

There should be no functionality change resulting from this patch.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Dave Jones <davej@redhat.com>