frappy_mlz: fix one-off error in barcode reader

cut of one byte too much in barcode decode

Change-Id: I5f1f8475f197b13af836d685dc6da5a9ee824dc2
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/31728
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

frappy_mlz/zebra.py

@@ -176,7 +176,7 @@ class ZebraIO(BytesIO):
         if buf[1] != expected_op:
             raise CommunicationFailedError('got op %r, expected %r' %
                                        (buf[0], expected_op))
-        return buf[4:-2]
+        return buf[3:-2]
 
     def _ssi_comm(self, op, data):
         self._ssi_send(op, data)