ldmvsw: unregistering netdev before disable hardware

When running LDom binding/unbinding test, kernel may panic
in ldmvsw_open(). It is more likely that because we're removing
the ldc connection before unregistering the netdev in vsw_port_remove(),
we set up a window of time where one process could be removing the
device while another trying to UP the device. This also sometimes causes
vio handshake error due to opening a device without closing it completely.
We should unregister the netdev before we disable the "hardware".

Orabug: 25980913, 25925306

Signed-off-by: Thomas Tai <thomas.tai@oracle.com>
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>