finsh: unregister rx_indicate when closing the device

Because the device could still remain opened when closed by finsh, the
old rx_indicate is useless for finsh. Some buggy driver will still
generate rx_indicate even after the device has been closed. So FinSh
should unregister the rx_indicate when releasing the old device.
Merge pull request #158 from grissiom/finsh-fix

finsh: unregister rx_indicate when closing the device