iio: adc: ad7606: don't use address field

iio: adc: ad7606: don't use address field

Drop use of the address field in the ad7606 driver. This field was
duplicating the same info that was already in the channel and scan_index
fields. This is one less thing to have to hold in your mind when reading
the code.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250318-iio-adc-ad7606-improvements-v2-7-4b605427774c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>