scsi: iscsi: Declare SCSI host template const

Make it explicit that the SCSI host template is not modified.

Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230322195515.1267197-50-bvanassche@acm.org


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>