scripts/decodetree.py: fix reference to attributes

Signed-off-by: NCleber Rosa <crosa@redhat.com>
Message-Id: <20181004161852.11673-9-crosa@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

scripts/decodetree.py

@@ -298,7 +298,7 @@ class Field:
             s = 's'
         else:
             s = ''
-        return str(pos) + ':' + s + str(len)
+        return str(self.pos) + ':' + s + str(self.len)
 
     def str_extract(self):
         if self.sign: