iio: adc: ad7192: move ad7192_of_match table closer to the end of the file

The change is more cosmetic. There is no need to reference this table in
the probe function since 'of_device_get_match_data' is used, which obtains
this information from the driver object.

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