hwmon: (ntc_thermistor) Ensure iio channel is of type IIO_VOLTAGE

When configured via device tree, the associated iio device needs to be
measuring voltage for the conversion to resistance to be correct.
Return -EINVAL if that is not the case.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/ntc_thermistor.c

@@ -239,8 +239,10 @@ static struct ntc_thermistor_platform_data *
 ntc_thermistor_parse_dt(struct platform_device *pdev)
 {
 	struct iio_channel *chan;
+	enum iio_chan_type type;
 	struct device_node *np = pdev->dev.of_node;
 	struct ntc_thermistor_platform_data *pdata;
+	int ret;
 
 	if (!np)
 		return NULL;
@@ -253,6 +255,13 @@ ntc_thermistor_parse_dt(struct platform_device *pdev)
 	if (IS_ERR(chan))
 		return ERR_CAST(chan);
 
+	ret = iio_get_channel_type(chan, &type);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	if (type != IIO_VOLTAGE)
+		return ERR_PTR(-EINVAL);
+
 	if (of_property_read_u32(np, "pullup-uv", &pdata->pullup_uv))
 		return ERR_PTR(-ENODEV);
 	if (of_property_read_u32(np, "pullup-ohm", &pdata->pullup_ohm))