[imx6] Fix VP8 timestamping

Hi,

I had a look at the remaining patches that may have been forgotten from the old xbmc-imx6 repo with helix release in approach.

That one comes from https://github.com/xbmc-imx6/xbmc/issues/52.
It is not so clean because I think the proper way would be to include the library libfslvpuwrap in kodi repo to be able to solve a few annoying bugs, or at least inconsistent behavior, this library exhibits.
Moreover it would be easier to build for android don't you think @koying ?
So if you agree, I can begin to prepare this library inclusion and then we will push in it the few fixes that may be useful to have a cleaner generic imx6 decode function...

By the time, this 3 lines patch is likely to avoid some reports from users who are not so happy with imx6 and VP8 codec for helix release...