GDAC Tuning returns unrealistic values for diff FE when good pixel mask is applied

@fhinterk observed that GDAC tuning does not work correctly for diff FE with good pixel mask applied using SP quads. The tuning procedure will return VTH1_DIFF value is 512 with mean occupancy 0 for every chip (see attached logfile chip_0.log). If good pixel mask is not applied, GDAC tuning for diff FE works fine (chip_3.log).

I double checked this with common quads and observed the same.