mxfdec: set audio timebase to 1/samplerate

Fixes sync in some samples (e.g. bugs 7581 and 8374 in VLC).
Based on a commit by Matthieu Bouron <matthieu.bouron@gmail.com>

Reported-by: Jean-Baptiste Kempf <jb@videolan.org>
CC: libav-stable@libav.org