ACPI / cpuidle: avoid assigning signed errno to acpi_status

It's incorrect to assign a signed value to an unsigned acpi_status
variable. This patch fixes it by using a signed integer to return
error values.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>