[SCSI] sd: Correctly handle 6-byte commands with DIX

DIF does not work with 6-byte commands so we previously ignored those
commands when preparing a request.  However, DIX does not need
RDPROTECT/WRPROTECT to be set and 6-byte commands are consequently
perfectly valid in host-only mode.

This patch fixes a problem where we would set the wrong DIX operation
when issuing commands to a legacy disk.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>