scsi: cleanup switch in scsi_adjust_queue_depth

While checking what scsi_adjust_queue_depth() did I thought its switch
statement could be clearer:

   - remove redundant assignment (to sdev->queue_depth)
   - re-order cases (thus removing the fall-through)

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Robert Elliott <elliott@hp.com>
Tested-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>