staging: iio: adt7316: use correct variable in DAC_internal_Vref read

The dac internal vref settings are part of the ldac config register rather
than the dac config register. Change the variable being used so the read
returns the correct result.

Whilst a fix, it is for a driver in heavy churn so do not backport
to stable without taking considerable care and testing.

Signed-off-by: Jeremy Fertic <jeremyfertic@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>