cpufreq: stats: rename 'struct cpufreq_stats' objects as 'stats'

Currently we name objects of 'struct cpufreq_stats' as 'stat' and 'stats'.
Use 'stats' to make it consistent.

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