Few functions in ivas_stereo_mdct_core_dec converted to fxd

Fixed point related changes done for functions:
[x] stereo_mdct_init_bands
[x] stereo_mdct_init_igf_start_band
[x] splitAvailableBits
[x] initMdctStereoDecData
[x] apply_dmx_weights
[x] stereo_mdct_dec_stereo