ACPI / LPSS: set PM domain via helper setter

The commit 989561de ("PM / Domains: add setter for dev.pm_domain") changed
acpi_lpss.c module to use PM domain setter, though it missed one assignment.
Add it here.

Fixes: 989561de (PM / Domains: add setter for dev.pm_domain)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_lpss.c

@@ -880,7 +880,7 @@ static int acpi_lpss_platform_notify(struct notifier_block *nb,
 		break;
 	case BUS_NOTIFY_DRIVER_NOT_BOUND:
 	case BUS_NOTIFY_UNBOUND_DRIVER:
-		pdev->dev.pm_domain = NULL;
+		dev_pm_domain_set(&pdev->dev, NULL);
 		break;
 	case BUS_NOTIFY_ADD_DEVICE:
 		dev_pm_domain_set(&pdev->dev, &acpi_lpss_pm_domain);