Small cleanup in LTC reader : use libltc function to convert LTC frame to timecode

Dear Joker developers,

Here is a small modification to the LTC reader to use a libltc function instead of converting the LTC timecode
by hand. The current code is right, but the function provided by libltc is a little clearer. Using this function also
allows to rely less on libltc internal timecode representation.

Thanks for your attention !