FEC errors on firefly links
We see a non-negligible FEC error rate on links that go through the firefly FMC extension card. Attenuators only make the issue worse, but can test some of the VTRx+ settings: https://gitlab.cern.ch/cms-etl-electronics/module_test_sw/-/blob/master/tamalero/VTRX.py?ref_type=heads#L179