datapaths: Fix pcap_switch with pcap ports
datapaths: Fix pcap_switch with pcap ports

I'm not sure how this has been broken for so long.  Maybe my fix is
wrong, but it seems right and seems to make things work, so...

MurphyMc/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox