[media] uvcvideo: Set error_idx properly for S_EXT_CTRLS failures

The uvc_set_ctrl() calls don't write to the hardware. A failure at that
point thus leaves the device in a clean state, with no control modified.
Set the error_idx field to the count value to reflect that, as per the
V4L2 specification.
TRY_EXT_CTRLS is unchanged and the error_idx field must always be set to
the failed control index in that case.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/uvc/uvc_v4l2.c

@@ -685,7 +685,8 @@ static long uvc_v4l2_do_ioctl(struct file *file, unsigned int cmd, void *arg)
 			ret = uvc_ctrl_set(chain, ctrl);
 			if (ret < 0) {
 				uvc_ctrl_rollback(handle);
-				ctrls->error_idx = i;
+				ctrls->error_idx = cmd == VIDIOC_S_EXT_CTRLS
+						 ? ctrls->count : i;
 				return ret;
 			}
 		}