Bugfix: assign correct FED IDs if some FEDs are not in the DAQ

Fixes a bug that manifests when some FEDs are out of the DAQ. The data integrity plots of the first n FEDs where filled no matter what werethe actual FEDs being read out. The fix solves this problem by reading the FED ID from the FED itself and not trying to (wrongly) guess it. A cleaner fix would involve not passing anymore the "id" variable to the processFED method (see FIXME): since it involves changing the interface of a public method, I defer this change to the next CMSSW version.