ACPI: scan: Rearrange acpi_dev_get_first_consumer_dev_cb()

Make acpi_dev_get_first_consumer_dev_cb() a bit more straightforward
and rewrite the comment in it.

No functional impact.

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