Changed code to allow comments if only source:maxspeed is set, as per osmose-qa#2506 (comment)

Author: wolfbert

plugins/TagFix_Maxspeed_AT.py

@@ -173,7 +173,7 @@ def way(self, data, tags, nds):
             # Error: source:maxspeed is invalid
             if source_maxspeed in self.valid_maxspeed_types.keys():
                 valid_type = source_maxspeed
-            else:
+            elif source_maxspeed.startswith('AT:') or source_maxspeed in {'zone', 'walk'}:
                 err.append({'class': 4,
                             'text': T_('Invalid source:maxspeed: `{0}`', source_maxspeed)})