cpufreq: persistent_stats: Use correct CPU to get freq table

If a policy manages multiple CPUs, frequency table might only be
available on policy->cpu. Use the correct CPU to obtain frequency
table.

Change-Id: I3a52bb88f64df70e32e511a37d8364a0f62b507c
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>