[media] uvcvideo: Work around buggy Logitech C920 firmware

The uvcvideo webcam driver exposes the v4l2 control "Exposure (Absolute)"
which allows the user to control the exposure time of the webcam,
essentially controlling the brightness of the received image.  By default
the webcam automatically adjusts the exposure time automatically but the
if you set the control "Exposure, Auto"="Manual Mode" the user can fix
the exposure time.

Unfortunately it seems that the Logitech C920 has a firmware bug where
it will forget that it's in manual mode temporarily during initialisation.
This means that the camera doesn't respect the exposure time that the user
requested if they request it before starting to stream video.  They end up
with a video stream which is either too bright or too dark and must reset
the controls after video starts streaming.

This patch introduces the quirk UVC_QUIRK_RESTORE_CTRLS_ON_INIT which
causes the cached controls to be re-uploaded to the camera immediately
after initialising the camera.  This quirk is applied to the C920 to work
around this camera bug.

Signed-off-by: William Manley <will@williammanley.net>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/usb/uvc/uvc_video.c

@@ -1678,6 +1678,12 @@ static int uvc_init_video(struct uvc_streaming *stream, gfp_t gfp_flags)
 		}
 	}
 
+	/* The Logitech C920 temporarily forgets that it should not be adjusting
+	 * Exposure Absolute during init so restore controls to stored values.
+	 */
+	if (stream->dev->quirks & UVC_QUIRK_RESTORE_CTRLS_ON_INIT)
+		uvc_ctrl_restore_values(stream->dev);
+
 	return 0;
 }