v4l2: v4l2_camera_proxy, v4l2_camera: Check return values of read/write

The return value of the write to the eventfd (to signal POLLIN) from
V4L2Camera and the read from the eventfd (to clear POLLIN) from
V4L2CameraProxy was ignored. Check the return value, and print an error

Reported-by: Coverity CID=290743
Reported-by: Coverity CID=290744
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
2 files changed