USB: serial: opticon: add chars_in_buffer() implementation

Add a chars_in_buffer() implementation so that the tty layer will wait
for outgoing buffered data to be drained when needed (e.g. on final
close()).

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