iio: ad5064: Fix regulator handling

The correct way to handle errors returned by regualtor_get() and friends is
to propagate the error since that means that an regulator was specified,
but something went wrong when requesting it.

For handling optional regulators, e.g. when the device has an internal
vref, regulator_get_optional() should be used to avoid getting the dummy
regulator that the regulator core otherwise provides.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>