Some drivers such as aha1542 and aic7xxx_old will call scsi_register() and
then, if some succeeding operations fails they will call scsi_unregister(),
without an intervening scsi_set_host().

This causes an oops in scsi_put_device(), because kobj->parent is NULL.

In other words, scsi_register() immediately followed by scsi_unregister()
is guaranteed to oops.

The patch makes scsi_host_dev_release() more robust against this usage
pattern.