usb: gadget: f_rndis: Avoid to use ERROR macro if cdev can be null

The udc_irq service runs the isr_tr_complete_handler which in turn
"nukes" the endpoints, including a call to rndis_response_complete,
if appropriate. If the rndis_msg_parser fails here, an error will
be printed using a dev_err call (through the ERROR() macro).

However, if the usb cable was just disconnected the device (cdev)
might not be available and will be null. Since the dev_err macro will
dereference the cdev pointer we get a null pointer exception.
Reviewed-by: Radovan Lekanovic <radovan.lekanovic@sonymobile.com>
Signed-off-by: Truls Bengtsson <truls.bengtsson@sonymobile.com>
Signed-off-by: Oskar Andero <oskar.andero@sonymobile.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_rndis.c

@@ -447,14 +447,13 @@ static void rndis_response_complete(struct usb_ep *ep, struct usb_request *req)
 static void rndis_command_complete(struct usb_ep *ep, struct usb_request *req)
 {
 	struct f_rndis			*rndis = req->context;
-	struct usb_composite_dev	*cdev = rndis->port.func.config->cdev;
 	int				status;
 
 	/* received RNDIS command from USB_CDC_SEND_ENCAPSULATED_COMMAND */
 //	spin_lock(&dev->lock);
 	status = rndis_msg_parser(rndis->config, (u8 *) req->buf);
 	if (status < 0)
-		ERROR(cdev, "RNDIS command error %d, %d/%d\n",
+		pr_err("RNDIS command error %d, %d/%d\n",
 			status, req->actual, req->length);
 //	spin_unlock(&dev->lock);
 }