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: 260442cc ("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>

drivers/iio/adc/ad7091r-base.c

@@ -406,7 +406,14 @@ int ad7091r_probe(struct device *dev, const char *name,
 	if (IS_ERR(st->vref)) {
 		if (PTR_ERR(st->vref) == -EPROBE_DEFER)
 			return -EPROBE_DEFER;
+
 		st->vref = NULL;
+		/* Enable internal vref */
+		ret = regmap_set_bits(st->map, AD7091R_REG_CONF,
+				      AD7091R_REG_CONF_INT_VREF);
+		if (ret)
+			return dev_err_probe(st->dev, ret,
+					     "Error on enable internal reference\n");
 	} else {
 		ret = regulator_enable(st->vref);
 		if (ret)

drivers/iio/adc/ad7091r-base.h

@@ -8,6 +8,8 @@
 #ifndef __DRIVERS_IIO_ADC_AD7091R_BASE_H__
 #define __DRIVERS_IIO_ADC_AD7091R_BASE_H__
 
+#define AD7091R_REG_CONF_INT_VREF	BIT(0)
+
 /* AD7091R_REG_CH_LIMIT */
 #define AD7091R_HIGH_LIMIT		0xFFF
 #define AD7091R_LOW_LIMIT		0x0