openvswitch: fix vport-netdev unregister

The combination of two commits:
commit 8e4e1713
("openvswitch: Simplify datapath locking.")
commit 2537b4dd
("openvswitch:: link upper device for port devices")

introduced a bug where upper_dev wasn't unlinked upon
netdev_unregister notification

The following steps:

  modprobe openvswitch
  ovs-dpctl add-dp test
  ip tuntap add dev tap1 mode tap
  ovs-dpctl add-if test tap1
  ip tuntap del dev tap1 mode tap

are causing multiple warnings:

[   62.747557] gre: GRE over IPv4 demultiplexor driver
[   62.749579] openvswitch: Open vSwitch switching datapath
[   62.755087] device test entered promiscuous mode
[   62.765911] device tap1 entered promiscuous mode
[   62.766033] IPv6: ADDRCONF(NETDEV_UP): tap1: link is not ready
[   62.769017] ------------[ cut here ]------------
[   62.769022] WARNING: CPU: 1 PID: 3267 at net/core/dev.c:5501 rollback_registered_many+0x20f/0x240()
[   62.769023] Modules linked in: openvswitch gre vxlan ip_tunnel libcrc32c ip6table_fil...