functions.php

<?php
/**
 * Apply Dropdown Menu Class to Primary Only
 *
 * @author Bill Erickson
 * @link https://gist.github.com/billerickson/6700212
 *
 * @param array $menus, theme locations
 * @return array $menus
 */
function be_dropdown_class_on_primary( $menus ) {
  return array( 'primary' );
}
add_filter( 'dropdown_menu_class_menus', 'be_dropdown_class_on_primary' );