USB: serial: opticon: stop all I/O on close()

Make sure to stop any submitted write URBs on close().

Note that the tty layer will wait up to 30 seconds for the buffers to
drain before close() is called.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>