Set the frame size when decoding DTS audio.

This has the side effect of fixing the computation of timestamps for DTS-HD in compute_pkt_fields.  Since frame_size is
not currently set, the duration of a frame is being guessed based on the streams bitrate.  But for DTS-HD, the bitrate
currently used is the rate of the DTS core which is much different than the whole DTS-HD stream and leads to a wildly
inaccurate frame duration estimate.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>