usb: dwc3: gadget: use udc-core's reset notifier

Replace usb_gadget_driver's disconnect with udc-core's reset notifier at
bus reset handler.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>