iio: adc: ad7173: remove unused field

Remove the unused chan_reg field from struct ad7173_channel. This was
set but never read.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241104-iio-adc-ad7173-remove-unused-field-v1-1-da9500a48750@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>