bug fix: member field EthernetChannel::m_nDevices has to be initialized

Field EthernetChannel::m_nDevices was not initialized before and caused the code to failed on this assert:

assert failed. cond="m_nDevices < N_DEVICES", msg="Only two devices permitted", file=ethernet-channel.cc, line=82
terminate called without an active exception 

Now it seems to work fine.