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>