iio: adc: ad9467: don't allow reading vref if not available

If there's only one possible scale, there's no way to change the Vref
select in the device so avoid reading the register in ad9467_get_scale().
In this case, it makes no sense to provide the .read_available()
callback nor allowing for writing the scale attribute.

Note this is in preparation for supporting a new device that only has
one possible scale.

Signed-off-by: Nuno Sa <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jonathan Cameron <[email protected]>

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