UI: speed_up doubles the duration_ms

[heshpdx]

This is an easy one to fix for anyone wanting to contribute. (I can get to it later if nobody else does).

If one supplies -su as a parameter alongside -d, then whatever value is passed to -d is effectively doubled. So if you specify -d 10000 -su to process the first ten seconds of audio at twice the rate, whisper.cpp will not exit until it completes twenty seconds of transcription.