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>