USB: serial: io_ti: drop closing_wait module parameter

Now that all USB serial drivers supports setting the closing_wait
parameter through TIOCSSERIAL (setserial) it's time to drop the
corresponding io_ti module parameter.

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