scsi: never drop to untagged mode during queue ramp down

Dropping to untagged mode when ramping down a queue due to QUEUE FULL
events has two problems:

 - nothing in the midlayer or drivers ever moves back to tagged mode
   during queue ramp up.
 - cmd_per_lun isn't the untagged queue depth for many modern drivers
   that can handle multiple untagged commands, and this is the only
   place in the midlayer assuming that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>