hwmon: (ntc_thermistor) Use library interpolation

hwmon: (ntc_thermistor) Use library interpolation

The kernel has a helper function for linear interpolation so
use it. It incidentally makes the code easier to read as well.

Tested on the ST-Ericsson HREFv60plus hardware reference design
with two thermistors forming a thermal zone.

Cc: Peter Rosin <peda@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210704222014.12058-1-linus.walleij@linaro.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>