staging:iio:ad7887: Rework regulator handling

Rework the regulator handling of the ad7887 driver to match more closely what we
do for other drivers. Only request the regulator if a external reference is
used, but treat it as an error if requesting the regulator fails. Also remove
the possibility to specify the reference voltage via platform data and always
use the regulator for this.

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