[media] uvcvideo: Fix V4L2 button controls that share the same UVC control

The Logitech pan/tilt reset UVC control contains two V4L2 button
controls to reset pan and tilt. As the UVC control is not marked as
auto-update, the button bits are set but never reset. A pan reset that
follows a tilt reset would thus reset both pan and tilt.

Fix this by not caching the control value of write-only controls. All
standard UVC controls are either readable or auto-update, so this will
not cause any regression and will not result in extra USB requests.
Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/uvc/uvc_ctrl.c

@@ -1351,9 +1351,12 @@ static int uvc_ctrl_commit_entity(struct uvc_device *dev,
 
 		/* Reset the loaded flag for auto-update controls that were
 		 * marked as loaded in uvc_ctrl_get/uvc_ctrl_set to prevent
-		 * uvc_ctrl_get from using the cached value.
+		 * uvc_ctrl_get from using the cached value, and for write-only
+		 * controls to prevent uvc_ctrl_set from setting bits not
+		 * explicitly set by the user.
 		 */
-		if (ctrl->info.flags & UVC_CTRL_FLAG_AUTO_UPDATE)
+		if (ctrl->info.flags & UVC_CTRL_FLAG_AUTO_UPDATE ||
+		    !(ctrl->info.flags & UVC_CTRL_FLAG_GET_CUR))
 			ctrl->loaded = 0;
 
 		if (!ctrl->dirty)