usb: gadget: langwell_udc: add usb test mode support

This patch adds test mode support for Langwell gadget driver.

Signed-off-by: Henry Yuan <hang.yuan@intel.com>
Signed-off-by: Andy Luo <yifei.luo@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>