iio: ad_sigma_delta: select channel when reading register

The desired channel has to be selected in order to correctly fill the
buffer with the corresponding data.
The `ad_sd_write_reg()` already does this, but for the
`ad_sd_read_reg_raw()` this was omitted.

Fixes: af300848 ("iio:adc: Add common code for ADI Sigma Delta devices")
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad_sigma_delta.c

@@ -121,6 +121,7 @@ static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
 	if (sigma_delta->info->has_registers) {
 		data[0] = reg << sigma_delta->info->addr_shift;
 		data[0] |= sigma_delta->info->read_mask;
+		data[0] |= sigma_delta->comm;
 		spi_message_add_tail(&t[0], &m);
 	}
 	spi_message_add_tail(&t[1], &m);