iio: ad7793: Fix the serial interface reset

BugLink: http://bugs.launchpad.net/bugs/1724783

commit 7ee3b7eb upstream.

The serial interface can be reset by writing 32 consecutive 1s to the device.
'ret' was initialized correctly but its value was overwritten when
ad7793_check_platform_data() was called. Since a dedicated reset function
is present now, it should be used instead.

Fixes: 2edb769d ("iio:ad7793: Add support for the ad7798 and ad7799")
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/iio/adc/ad7793.c

@@ -257,7 +257,7 @@ static int ad7793_setup(struct iio_dev *indio_dev,
 	unsigned int vref_mv)
 {
 	struct ad7793_state *st = iio_priv(indio_dev);
-	int i, ret = -1;
+	int i, ret;
 	unsigned long long scale_uv;
 	u32 id;
 
@@ -266,7 +266,7 @@ static int ad7793_setup(struct iio_dev *indio_dev,
 		return ret;
 
 	/* reset the serial interface */
-	ret = spi_write(st->sd.spi, (u8 *)&ret, sizeof(ret));
+	ret = ad_sd_reset(&st->sd, 32);
 	if (ret < 0)
 		goto out;
 	usleep_range(500, 2000); /* Wait for at least 500us */