ide: use 'drive->dn & 1' instead of drive->select.b.unit

ide: use 'drive->dn & 1' instead of drive->select.b.unit

* Call ide_port_init_devices() in ide_host_register()
  also if 'struct ide_port_info *d' is not available.

* Init drive->dn in ide_port_init_devices() instead of
  ide_probe_port() so it is valid also in ->init_dev.

* Pass device number to ide_dev_apply_params().

* Use 'drive->dn & 1' instead of drive->select.b.unit.

There should be no functional changes caused by this patch.

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