ntb_netdev: remove from list on exit

commit 904435cf upstream.

The ntb_netdev device is not removed from the global list of devices
upon device removal.  If the device is re-added, the removal code would
find the first instance and try to remove an already removed device.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>