cpufreq: Fix a comment in cpufreq_policy_free

Make the comment in blocking_notifier_call_chain() easier to
understand.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>