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/libcamera/v4l2_subdevice.cpp

@@ -206,7 +206,7 @@ uint8_t V4L2SubdeviceFormat::bitsPerPixel() const
 	const auto it = formatInfoMap.find(mbus_code);
 	if (it == formatInfoMap.end()) {
 		LOG(V4L2, Error) << "No information available for format '"
-				 << toString() << "'";
+				 << *this << "'";
 		return 0;
 	}