iio: dac: ad5686: Don't set error code to voltage_uv

regulator_get_voltage() may return negative error code.
Add error checking to avoid setting error code to voltage_uv.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>