cpuidle: make __cpuidle_driver_init() return void

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

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