staging:iio:ad7476: Rework reference voltage handling

Slightly rework the reference voltage handling for the ad7476 driver. Now the only
way to specify a external reference voltage is to use the regulator API,
previously it was possible to use either platform_data or the regulator API. The
new way is more consistent with what other drivers do.

Also do not ignore errors when requesting the regulator, since this will cope
very poorly with e.g. deferred probing.

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