cpuidle: rearrange __cpuidle_register_device() to keep minimal exit points

This patch rearranges __cpuidle_register_device() a bit in order to
reduce the number of exit points in that function.

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