clean up EventFilter/HcalRawToDigi/plugins/HcalDigisProducerGPU.cc

[fwyzard]

1. clear host buffers once the copy has completed (so in produce()):

  // clear host buffers
  hf01_.clear();
  hf5_.clear();
  hf3_.clear();

2. add a runtime check that the "maxChannels" sizes are not overrun

3. check what is a good value