[.] FO Fix ProductSales::getBestSales() query for product default order 'date_upd'/'date_add'

Bug :
- Admin > Preferences > Products : Default order by : choose 'Product
Add date' (or 'Product modified date').
- open page '/index.php?controller=best-sales'

You should have the sql error (in _PS_MODE_DEV_ mode).

This commit fix that 'Column 'date_add' in order clause is ambiguous'
error.
just indentation