scsi: csiostor: drop serial_number usage

Use request tag instead of the serial number when printing out logging
messages.

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>