usb: renesas_usbhs: fixup gadget.dev.driver when udc_stop.

current renesas_usbhs is using new style udc_start/stop from
af1d7056
(usb: gadget: renesas: convert to new style).

But current renesas_usbhs driver didn't care about gadget.dev.driver
when udc_stop. it cause rmmod oops.
This patch care it.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/renesas_usbhs/mod_gadget.c

@@ -776,6 +776,7 @@ static int usbhsg_gadget_stop(struct usb_gadget *gadget,
 		return -EINVAL;
 
 	usbhsg_try_stop(priv, USBHSG_STATUS_REGISTERD);
+	gpriv->gadget.dev.driver = NULL;
 	gpriv->driver = NULL;
 
 	return 0;