libcamera: v4l2_subdevice: Silence warning for unknown metadata formats

Commit e297673e76 ("libcamera: v4l2_device: Adjust colorspace based on
pixel format") has introduced a warning when trying to convert a color
space from V4L2 to libcamera if the media bus code is unknown. This was
meant to catch unknown image formats, but turned out to be also
triggered for metadata formats.

Color spaces are not applicable to metadata formats, there should thus
be no warning. Fix it by skipping the color space translation and
returning std::nullopt directly if the kernel reports
V4L2_COLORSPACE_DEFAULT. This doesn't introduce any change in behaviour
other than getting rid of the warning, as the V4L2Device::toColorSpace()
function returns std::nullopt already in that case.

Fixes: e297673e76 ("libcamera: v4l2_device: Adjust colorspace based on pixel format")
Reported-by: Naushir Patuck <naush@raspberrypi.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Tested-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>

include/libcamera/internal/v4l2_subdevice.h

@@ -101,6 +101,9 @@ protected:
 private:
 	LIBCAMERA_DISABLE_COPY(V4L2Subdevice)
 
+	std::optional<ColorSpace>
+	toColorSpace(const v4l2_mbus_framefmt &format) const;
+
 	std::vector<unsigned int> enumPadCodes(unsigned int pad);
 	std::vector<SizeRange> enumPadSizes(unsigned int pad,
 					    unsigned int code);