serial: imx: default to half duplex rs485

The i.MX driver defaulted to full duplex rs485 which is rather
unusual and doesn't match the default implemented in other drivers.

So change the default to half duplex.

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