ide: factor out devices setup from ide_acpi_init()

* Factor out devices setup from ide_acpi_init() to
  ide_acpi_port_init_devices().

* Call ide_acpi_port_init_devices() in ide_device_add_all().

While at it:
* Remove no longer needed 'drive' field from struct ide_acpi_drive_link.

There should be no functionality changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>