iio: adc: ad7887: invert/rework external ref logic

This change inverts/reworks the logic to use an external reference via a
provided regulator.

Now the driver tries to obtain a regulator. If one is found, then it is
used. The rest of the driver logic already checks if there is a non-NULL
reference to a regulator, so it should be fine.

Signed-off-by: Alexandru Ardelean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad7887.c

···
 
 	st = iio_priv(indio_dev);
 
-	if (!pdata || !pdata->use_onchip_ref) {
-		st->reg = devm_regulator_get(&spi->dev, "vref");
-		if (IS_ERR(st->reg))
+	st->reg = devm_regulator_get_optional(&spi->dev, "vref");
+	if (IS_ERR(st->reg)) {
+		if (PTR_ERR(st->reg) != -ENODEV)
 			return PTR_ERR(st->reg);
 
+		st->reg = NULL;
+	}
+
+	if (st->reg) {
 		ret = regulator_enable(st->reg);
 		if (ret)
 			return ret;
···
 	/* Setup default message */
 
 	mode = AD7887_PM_MODE4;
-	if (!pdata || !pdata->use_onchip_ref)
+	if (!st->reg)
 		mode |= AD7887_REF_DIS;
 	if (pdata && pdata->en_dual)
 		mode |= AD7887_DUAL;

include/linux/platform_data/ad7887.h

···
  *	second input channel, and Vref is internally connected to Vdd. If set to
  *	false the device is used in single channel mode and AIN1/Vref is used as
  *	VREF input.
- * @use_onchip_ref: Whether to use the onchip reference. If set to true the
- *	internal 2.5V reference is used. If set to false a external reference is
- *	used.
  */
 struct ad7887_platform_data {
 	bool en_dual;
-	bool use_onchip_ref;
 };
 
 #endif /* IIO_ADC_AD7887_H_ */