netdev-bsd: Use underlying tap device on netdev_bsd_listen().

netdev-bsd: Use underlying tap device on netdev_bsd_listen().

This is a trivial port to netdev-bsd of Justin Pettit's netdev-linux
commit 33d82a56d (netdev-linux: Use underlying tap device on
netdev_linux_listen().), which had the commit message:

  Commit acf608 (ofproto-dpif: Use a single underlying datapath across
  multiple bridges.) broke connectivity to userspace datapath devices.  The
  code assumed the first caller to open a tap device with
  netdev_linux_open() wanted to write to it.  This commit moves that logic
  to when netdev_linux_listen() is called.

This fixes the userspace datapath on FreeBSD.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>