[non-BE][allow-regression] Resolve "Review code in stereo_dft_enc_compute_itd()"

• Related issues: #2561 (closed)
• Requested reviewers: @norvell or @janssontoftg

Reason why this change is needed

• Align low pass filter (cross) L/R power spectrum with floating-point code

Description of the change

• Align low pass filter (cross) L/R power spectrum with floating-point code

Affected operating points

• Affacts DTX/CNG for DFT Stereo. Non-BE, but fully interoperable.

Closes #2561 (closed)