Fixed warning in institution editing page.

Institution expiry field is only availble to site admins.
Signed-off-by: Ruslan Kabalin <ruslan.kabalin@luns.net.uk>

htdocs/admin/users/institutions.php

@@ -385,10 +385,10 @@ function institution_submit(Pieform $form, $values) {
     }
     $newinstitution->theme                        = (empty($values['theme']) || $values['theme'] == 'sitedefault') ? null : $values['theme'];
     if ($institution != 'mahara') {
-        $newinstitution->expiry                   = db_format_timestamp($values['expiry']);
         $newinstitution->defaultmembershipperiod  = ($values['defaultmembershipperiod']) ? intval($values['defaultmembershipperiod']) : null;
         if ($USER->get('admin')) {
             $newinstitution->maxuseraccounts      = ($values['maxuseraccounts']) ? intval($values['maxuseraccounts']) : null;
+            $newinstitution->expiry               = db_format_timestamp($values['expiry']);
         }
     }