cpufreq: Explain the kobject_put() in cpufreq_policy_alloc()

It may not be particularly clear why the kobject_put() after
failing kobject_init_and_add() in cpufreq_policy_alloc() is not
redundant, so add a comment to explain that.

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