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>