[FIX] check for less than instead of greater than

This fixes what seems to be gone wrong in 63b8fe0d:
```
git diff 63b8fe0d~1 63b8fe0d
diff --git a/openerp/osv/expression.py b/openerp/osv/expression.py
index c01f4c9..70a2841 100644
--- a/openerp/osv/expression.py
+++ b/openerp/osv/expression.py
@@ -1012,10 +1012,10 @@ class expression(object):
 
             else:
                 if field._type == 'datetime' and right and len(right) == 10:
-                    if operator in ('>', '>=', '='):
-                        right += ' 00:00:00'
-                    elif operator in ('<', '<='):
+                    if operator in ('>', '<='):
                         right += ' 23:59:59'
+                    else:
+                        right += ' 00:00:00'
                     push(create_substitution_leaf(leaf, (left, operator, right), working_model))
 
                 elif field.translate and right:
```