avformat/mp3dec: prefer "fast_seek" to TOC seek for CBR files.

"Fast seek" uses linear interpolation to find the position of the
requested seek time. For CBR this is more direct than using the
mp3 TOC and bypassing the TOC avoids problems with TOC precision.
(see https://crbug.com/545914#c13)

For VBR, fast seek is not precise, so continue to prefer the TOC
when available (the lesser of two evils).

Also, some re-ordering of the logic in mp3_seek to simplify and
give usetoc=1 precedence over fastseek flag.

Signed-off-by: wm4 <nfxjfg@googlemail.com>
