Harmonize GetPLCModeDecision_ivas_fx() and GetPLCModeDecision_fx()

Bug description

In the floating-point code, we have only one variant of

• GetPLCModeDecision_ivas_fx()
• GetPLCModeDecision_fx()

It should be feasable to harmonize these two functions.

It should be checked whether GetPLCModeDecision_ivas_fx() is inline with the floating-point counterpart. If yes, probably this variant could be used for FX. The used function should be named GetPLCModeDecision_fx().