iio: ad5686: fix optional reference voltage declaration

When not using the "_optional" function, a dummy regulator is returned
and the driver fails to initialize.

Signed-off-by: Urs Fässler <urs.fassler@bytesatwork.ch>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>