Resolve "Possible porting error in TDREND_REND_RenderSourceHRFilt_fx"

• Related issues: #2515 (closed)
• Requested reviewers: @norvell, @janssontoftg, or @demirkanik

Reason why this change is needed

• There seems to be a porting error.

Description of the change

• Fix error by changing to correct variable.

Affected operating points

• Probably affects output of all TDREND rendering.

Closes #2515 (closed)