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>

drivers/hwmon/ntc_thermistor.c

@@ -145,7 +145,7 @@ struct ntc_data {
 static int ntc_adc_iio_read(struct ntc_thermistor_platform_data *pdata)
 {
 	struct iio_channel *channel = pdata->chan;
-	unsigned int result;
+	s64 result;
 	int val, ret;
 
 	ret = iio_read_channel_raw(channel, &val);
@@ -155,10 +155,10 @@ static int ntc_adc_iio_read(struct ntc_thermistor_platform_data *pdata)
 	}
 
 	/* unit: mV */
-	result = pdata->pullup_uv * val;
+	result = pdata->pullup_uv * (s64) val;
 	result >>= 12;
 
-	return result;
+	return (int)result;
 }
 
 static const struct of_device_id ntc_match[] = {