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>