cpuidle: Fix menu_device->intervals type

Struct menu_device member intervals is declared as u32, but the value
stored is (unsigned) int. The type is changed to match the value being
stored.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>