ACPI: Fix acpi_pm_device_sleep_state()

Fix acpi_pm_device_sleep_state() to return the value returned
by _SxD if the device is supposed to wake up the system from
given sleep state and the evaluation of _SxW fails (e.g. _SxW
is not present).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>