commit adba6575 upstream.

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>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>