Harmonize generate_comfort_noise_dec_hf_fx() and generate_comfort_noise_dec_hf_ivas_fx()

Bug description

At the first glance it seems that the functions generate_comfort_noise_dec_hf_fx() and generate_comfort_noise_dec_hf_ivas_fx() are only different by code which is controlled by cng_coh_flag.

It should be possible to harmonize these functions.