Fix two problems related to list_head's (there are more, wip).
Second bug was uncovered by wli's list_head debugging patch, thanks wli!

- Remove ata_unused list and use &idedefault_driver->drives only,
  fixes list corruption (ata_unused will be later ressurected for hotplug).

- Do not add same device twice to &idedefault_driver->drives, triggered
  by first calling ide_unregister_subdriver() and later ata_attach().