ACPI / PM: Replace ACPI_STATE_D3 with ACPI_STATE_D3_COLD in device_pm.c

The two symbols ACPI_STATE_D3 and ACPI_STATE_D3_COLD actually
represent the same number (4), but ACPI_STATE_D3 is slightly
ambigugous, because it may not be clear that it really means D3cold
and not D3hot at first sight.

Remove that ambiguity from drivers/acpi/device_pm.c by making it
use ACPI_STATE_D3_COLD everywhere instead of ACPI_STATE_D3.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>