iio: adc: ad9467: Fix the "don't allow reading vref if not available" case

iio: adc: ad9467: Fix the "don't allow reading vref if not available" case

The commit in Fixes adds a special case when only one possible scale is
available.
If several scales are available, it sets the .read_avail field of the
struct iio_info to ad9467_read_avail().

However, this field already holds this function pointer, so the code is a
no-op.

Use another struct iio_info instead to actually reflect the intent
described in the commit message. This way, the structure to use is selected
at runtime and they can be kept as const.

This is safer because modifying static structs that are shared between all
instances like this, based on the properties of a single instance, is
asking for trouble down the road.

Fixes: b92f94f74826 ("iio: adc: ad9467: don't allow reading vref if not available")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/cc65da19e0578823d29e11996f86042e84d5715c.1733503146.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>