hwmon: (ntc_thermistor) Avoid math overflow

The ntc thermistor code was doing math whose temporary result might
have overflowed 32-bits.  We need some casts in there to make it safe.

In one example I found:
- pullup_uV: 1800000
- result of iio_read_channel_raw: 3226
- 1800000 * 3226 => 0x15a1cbc80

Signed-off-by: Doug Anderson <dianders@chromium.org>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>