staging:iio:ad7793: Rework regulator handling

Rework the regulator handling of the driver to match more closely what we do in
other drivers. Make the regulator non-optional if a external reference is used.
Also dispose the option of specifying the reference voltage via platform data.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>