intel_idle: Fix a helper function's return value.

The function, intel_idle_cpuidle_driver_init, delivers no error codes
at all.  This patch changes the function to return 'void' instead of
returning zero.

Signed-off-by: Richard Cochran <rcochran@linutronix.de>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>