MLK-22366-2 usb: cdns3: gadget: fix the hot plug and function switch issue

MLK-22366-2 usb: cdns3: gadget: fix the hot plug and function switch issue

- If the UDC is not started, we need not to access register at .udc_stop
- Clear all priv_ep flags except EP_CLAIMED which should be cleared at
.udc_stop
- Clear warning message that the .ep_disable may be called twice, one
from class driver, one from UDC driver.

Reviewed-by: Jun Li <jun.li@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>