scsi: mvumi: use request tag instead of serial_number

Use the request tag for logging instead of the scsi command serial number.

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