scsi-bus: remove bogus assertion

This assertion is invalid, because get_sg_list can return an
empty sg-list even for commands that transfer no data (such
as SYNCHRONIZE CACHE).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-bus.c

@@ -101,7 +101,6 @@ static void scsi_dma_restart_bh(void *opaque)
                 scsi_req_continue(req);
                 break;
             case SCSI_XFER_NONE:
-                assert(!req->sg);
                 scsi_req_dequeue(req);
                 scsi_req_enqueue(req);
                 break;