Change port-ref definition/leaf found within port-map list to use
if:interface-ref (or a leafref type to an interface), instead of the
using a port-number-type. Using the ietf-interface name would improve
the YANG interface's usability.
A 'must' statement should be used to limit the if:interface-ref's
range, e.g:
  typedef bridge-port-ref {
    type if:interface-ref;
    must "/if:interfaces/if:interface[if:name=current()]/if:type = 'ianaift:ethernetCsmacd'";
  }