ata_scsi_slave_destroy — SCSI device is about to be destroyed
|struct scsi_device * sdev|
sdev is about to be destroyed for hot/warm unplugging. If
this unplugging was initiated by libata as indicated by NULL
dev->sdev, this function doesn't have to do anything.
Otherwise, SCSI layer initiated warm-unplug is in progress.
Clear dev->sdev, schedule the device for ATA detach and invoke
LOCKING: Defined by SCSI layer. We don't really care.