cpufreq: stats: Improve module description string

The MODULE_DESCRIPTION() string is just too long and then is broken into
multiple lines just to make checkpatch happy.

Rewrite it to make it more precise.

Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>