cpufreq: prevent lockup on reading scaling_available_frequencies

When scaling_available_frequencies is read on an offlined cpu, then
either lockup or junk values are displayed. This is caused by
freed freq_table, which policy is using.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>