gt68xx: Added temporary fix to avoid stop scan during cancel.

Calling this function upsets some scanners in this family.
This temporary sticking plaster permits the Mustek 1248UB scanner to
scan after cancelling. We need to look into this much more when a
scanner becomes available for investigation.

backend/gt68xx.c

@@ -2329,7 +2329,14 @@ sane_cancel (SANE_Handle handle)
       /* some scanners don't like this command when cancelling a scan */
       sanei_usb_set_timeout (SHORT_TIMEOUT);
       gt68xx_device_fix_descriptor (s->dev);
-      gt68xx_scanner_stop_scan (s);
+
+      // FIXME: Temporary sticking plaster fix for models that cannot scan after calling this
+      // function to cancel. [RL]
+      if (s->dev->model->command_set != &mustek_gt6816_command_set)
+        {
+          gt68xx_scanner_stop_scan (s);
+        }
+
       sanei_usb_set_timeout (LONG_TIMEOUT);
 
       if (s->dev->model->flags & GT68XX_FLAG_SHEET_FED)