scsi: don't add scsi command result bytes

Some drivers are ADDing the scsi command's result bytes instead of ORing
them.

While this can produce correct results it has unexpected side effects.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>