[non-BE] Resolve "FD CNG buffer is not updated"

• Related issues: #2262 (closed)
• Requested reviewers: @multrus

Reason why this change is needed

• Buffer is currently not properly initialized.

Description of the change

• Use 16 bit buffer (from EVS) instead of otherwise unsed 32 bit buffer

Affected operating points

• IVAS bitrate switching with DTX on

Closes #2262 (closed)