Resolve "Review integer divisions in ivas_mct_enc_fx()"

• Related issues: #2531 (closed)
• Requested reviewers: @kiene or @fotopoulou

Reason why this change is needed

• Calculation of cp_bitrate using div_l() is obviously wrong; affects bitrate switching only

Description of the change

• Fall back to iDiv_and_mod_32(), as used at other spots in the same file.

Affected operating points

• Apparently the output is BE.

Closes #2531 (closed)