ACPI / scan: Do not use dummy HID for system bus ACPI nodes

At one point acpi_device_set_id() checks if acpi_device_hid(device)
returns NULL, but that never happens, so system bus devices with an
empty list of PNP IDs are given the dummy HID ("device") instead of
the "system bus HID" ("LNXSYBUS").  Fix the code to use the right
check.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: <stable@vger.kernel.org>