scsi: ibmvfc: ibmvscsi: ibmvscsi_tgt: constify vio_device_id

vio_device_id are not supposed to change at runtime. All functions
working with vio_device_id provided by <asm/vio.h> work with const
vio_device_id. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>