Use devclass_find_free_unit() in ata_pci_add_child() because no more implicit auto-scanning up present in device_add_child(). Functionally it is fully equivalent to old code.
Use devclass_find_free_unit() in ata_pci_add_child() because no more
implicit auto-scanning up present in device_add_child(). Functionally it is
fully equivalent to old code.

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