wdp-xflow: Always report an up-to-date port list in wx_port_list().

wdp-xflow: Always report an up-to-date port list in wx_port_list().

Until now wx_port_list() has reported the cached list of ports kept in
wx->ports.  This won't work, however, in a followup patch when bridge.c
switches from using an xfif to using a wdp, because the bridge
reconfiguration code expects to add and delete ports and have the list
of ports returned by wx_port_list() updated immediately.