(LTC reader) Use ltc.h function to convert LTC frame to timecode

The current code is right, but libltc already provides a function
to achieve that which a little clearer. Using this function also
allows to rely less on libltc internal timecode representation.
Merge pull request #213 from tlecomte/tlLTCframe_to_time

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