iio: adc: ad7091r: Enable internal vref if external vref is not supplied

iio: adc: ad7091r: Enable internal vref if external vref is not supplied

The ADC needs a voltage reference to work correctly.
Users can provide an external voltage reference or use the chip internal
reference to operate the ADC.
The availability of an in chip reference for the ADC saves the user from
having to supply an external voltage reference, which makes the external
reference an optional property as described in the device tree
documentation.
Though, to use the internal reference, it must be enabled by writing to
the configuration register.
Enable AD7091R internal voltage reference if no external vref is supplied.

Fixes: 260442cc5be4 ("iio: adc: ad7091r5: Add scale and external VREF support")
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/b865033fa6a4fc4bf2b4a98ec51a6144e0f64f77.1703013352.git.marcelo.schmitt1@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>