iio: adc: ti-ads7950: Ensure CS is deasserted after reading channels

The ADS7950 requires that CS is deasserted after each SPI word. Before
commit e2540da8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce
CPU usage") the driver used a message with one spi transfer per channel
where each but the last one had .cs_change set to enforce a CS toggle.
This was wrongly translated into a message with a single transfer and
.cs_change set which results in a CS toggle after each word but the
last which corrupts the first adc conversion of all readouts after the
first readout.

Fixes: e2540da8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce CPU usage")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: David Lechner <david@lechnology.com>
Tested-by: David Lechner <david@lechnology.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210709101110.1814294-1-u.kleine-koenig@pengutronix.deSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads7950.c

@@ -568,7 +568,6 @@ static int ti_ads7950_probe(struct spi_device *spi)
 	st->ring_xfer.tx_buf = &st->tx_buf[0];
 	st->ring_xfer.rx_buf = &st->rx_buf[0];
 	/* len will be set later */
-	st->ring_xfer.cs_change = true;
 
 	spi_message_add_tail(&st->ring_xfer, &st->ring_msg);