Presta breaks if you set modification date as default search order

This fixes the following problem:
 - set modification date as the default sort order in Preferences -> Products
 - do a search
 - change sorting to some non-default order
 - change sorting to the default
 - watch Presta break

The problem is because date_upd is ambiguous  in the search query. The problem is not apparent because the search has position hardcoded as the default order (which I think should be fixed as well).