cpuidle: make __cpuidle_device_init() return void

The only value returned by __cpuidle_device_init() is 0, so it very
well may be a void function.  Make that happen.

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