cpufreq: Rearrange cpufreq_add_dev()

Reorganize the code in cpufreq_add_dev() to avoid using the ret
variable and reduce the indentation level in it.

No functional changes.

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