iio: adc: ad7298: rework external ref setup & remove platform data

iio: adc: ad7298: rework external ref setup & remove platform data

This change removes the old platform data for ad7298. It is only used to
provide whether to use an external regulator as a reference.

So, the logic is inverted a bit. The driver now tries to obtain a
regulator. If one is provided, then the external ref is used. The rest of
the logic should work as before.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201001141048.69050-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>