serial: fsl_lpuart: clear unsupported options in .rs485_config()

The struct serial_rs485 parameter is both input and output and is
supposed to hold the actually used configuration on return. So clear
unsupported settings.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>