cpufreq: Use clamp() helper macro to improve the code readability

The valid values of policy.{min, max} should be between 'min' and 'max',
so use clamp() helper macro to makes cpufreq_verify_within_limits() easier
to follow.

Signed-off-by: Liao Chang <liaochang1@huawei.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>