Make static DCCDataUnpacker::silentMode_ std::atomic

Given that DCCDatdaUnpacker::silentMode_ is a public global and
could be modified by any thread, this had to be changed to an
std::atomic<bool>.
Merge pull request #2777 from Dr15Jones/makeDCCDataUnpackerStaticThreadSafe

EventFilter/EcalRawToDigi -- Make static DCCDataUnpacker::silentMode_ std::atomic