cpufreq: Remove extra variables from cpufreq_add_dev_symlink()

We call cpufreq_cpu_get() in cpufreq_add_dev_symlink() to increase usage
refcount of policy, but not to get a policy for the given CPU.  So, we
don't really need to capture the return value of this routine.  We can
simply use policy passed as an argument to cpufreq_add_dev_symlink().

Moreover debug print is rewritten to make it more clear.

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