iio: adc: ad7606: fix reading unnecessary data from device

When a conversion result is being read from ADC, the driver reads the
number of channels + 1 because it thinks that IIO_CHAN_SOFT_TIMESTAMP
is also a physical channel. This patch fixes this issue.

Fixes: 2985a5d8 ("staging: iio: adc: ad7606: Move out of staging")
Reported-by: Robert Wörle <rwoerle@mibtec.de>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7606.c

@@ -85,7 +85,7 @@ static int ad7606_reg_access(struct iio_dev *indio_dev,
 
 static int ad7606_read_samples(struct ad7606_state *st)
 {
-	unsigned int num = st->chip_info->num_channels;
+	unsigned int num = st->chip_info->num_channels - 1;
 	u16 *data = st->data;
 	int ret;