avformat/dtsdec: switch to common frame header parsing function

This makes probing for regular DTS more strict because more header
fields are checked and values not supported by decoder are now rejected.

Also fixes an issue original code had with 14-bit streams: 96 bits of
header were expected, however only 84 bits were converted, which was not
enough to parse LFE flag.

Signed-off-by: James Almer <jamrial@gmail.com>
