[media] omap3isp: video: Set the buffer bytesused field at completion time

The v4l buffer bytesused field is a value that will be returned to
userspace when the buffer gets dequeued. As such it doesn't need to be
set early at buffer queue time. Move the assignment to buffer completion
in the omap3isp_video_buffer_next() function to prepare for the video
buffers queue refactoring.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>