ACPI: scan: Introduce acpi_fetch_acpi_dev()

Introduce acpi_fetch_acpi_dev() as a more reasonable replacement for
acpi_bus_get_device() and modify the code in scan.c to use it instead
of the latter.

No expected functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>