scsi: scsi_debug: Avoid PI being disabled when TPGS is enabled

It was not possible to enable both T10 PI and TPGS because they share
the same byte in the INQUIRY response. Logically OR the TPGS value
instead of using assignment.

Reported-by: Ritika Srivastava <ritika.srivastava@oracle.com>
Reviewed-by: Bart Van Assche <Bart.VanAssche@sandisk.com>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>