staging: iio: adc: ad7192: modify iio_chan_spec array

This patch changes the static const struct iio_chan_spec arrays
in a way that all the necessary attributes are set at
compile time. Now ad7192_channels_config only makes the
channels attribute of iio_dev point to the right array depending
on the chip.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>