cpufreq: Clarify the comment in cpufreq_set_policy()

One of the responsibility of the ->verify() callback is to make sure
that the policy's min frequency is <= max frequency as this isn't
guaranteed by the QoS framework which gave us those values.

Update the comment in cpufreq_set_policy() to clarify that.

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Minor changes of the new comment ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>