ACPI / scan: Clean up acpi_bus_get_parent()

Make acpi_bus_get_parent() more straightforward and remove an
unnecessary local variable ret from it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>