scsi: sr: constify sr_pm_ops structure

sr_pm_ops, of type struct dev_pm_ops, is never modified, so declare it
as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>