serial: imx: fix bogus dev_err

Only one of the two is really required, not both:
* have_rtscts or
* have_rtsgpio

In imx_rs485_config() this is done correctly, so RS485 is working,
just the error message is false.

Signed-off-by: Phil Eichinger <phil@zankapfel.net>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Fixes: b8f3bff0 ("serial: imx: Support common rs485 binding for RTS polarity"
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>