iio: adc: ad9467: wrap a axi-adc chip-info into a ad9467_chip_info type

There are 2 chip constants that can be added to the chip-info part. The
default output-mode and the VREF mask.

When adding new chips to this driver, these can be easily omitted, because
these also need to be updated in 2 switch statements.

However, if adding them in the chip-info constants, they are updated in a
single place and propagated in both switch statements.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200924080518.96410-2-alexandru.ardelean@analog.com