Resolve "USAN: implicit-signed-integer-truncation in FEC HQ phase ECU"

• Related issues:
• Requested reviewers: Jan Kiene

Reason why this change is needed

• USAN error

Description of the change

• better handling of limited 16 bit counter range
• Solution will strive to be BE
• (Solution might eventually result in a desired update of the correspondig) EVS-c-code

IN detail the solution was to remove "+=" in:

  acc_var += input _var,_

_for the case where both variables were int16_t ,

Essentially  it seems that there is (according to CLANG) a  risk of an intermediate int32_t representation being truncated when using "+=".

Affected operating points

When Phase Ecu is used (HQ-mode in EVS, IVAS Core)

Related to ticket #1179 (closed)