libcamera: Replace toString with operator<<() for format classes

Now that format classes implement the stream formatting operator<<(),
use it instead of the toString() function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/v4l2/v4l2_camera_proxy.cpp

@@ -327,7 +327,7 @@ int V4L2CameraProxy::tryFormat(struct v4l2_format *arg)
 	if (ret < 0) {
 		LOG(V4L2Compat, Error)
 			<< "Failed to negotiate a valid format: "
-			<< format.toString();
+			<< format;
 		return -EINVAL;
 	}