Fix for fft sizes in whisper.cpp

[goldwaving]

fft_out needs to be twice the frame_size, not the frame_step. It is resized in fft() anyway, but this change prevents an unnecessary reallocation.

n_fft must match the mel filter size, so it is best not to calculate it from frame_size.

We only need to get the magnitudes for half the spectrum since the other half is a mirror and not used in the mel filter loop later.