ACPI / PM: Use string "D3cold" to represent ACPI_STATE_D3_COLD

Make acpi_power_state_string() return "D3cold" as the string
representation of ACPI power state D3cold instead of "D3" returned
currently, which is confusing.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/device_pm.c

@@ -115,7 +115,7 @@ const char *acpi_power_state_string(int state)
 	case ACPI_STATE_D3_HOT:
 		return "D3hot";
 	case ACPI_STATE_D3_COLD:
-		return "D3";
+		return "D3cold";
 	default:
 		return "(unknown)";
 	}