hwmon: (isl28022) Fix integer overflow in power calculation on 32-bit

isl28022_read_power() computes:

  *val = ((51200000L * ((long)data->gain)) /
          (long)data->shunt) * (long)regval;

On 32-bit platforms, 'long' is 32 bits. With gain=8 and shunt=10000
(the default configuration):

  (51200000 * 8) / 10000 = 40960
  40960 * 65535 = 2,684,313,600

This exceeds LONG_MAX (2,147,483,647), resulting in signed integer
overflow.

Additionally, dividing before multiplying by regval loses precision
unnecessarily.

Use u64 arithmetic with div_u64() and multiply before dividing to
retain precision. The intermediate product cannot overflow u64
(worst case: 51200000 * 8 * 65535 = 26843136000000). Power is
inherently non-negative, so unsigned types are the natural fit.
Cap the result to LONG_MAX before returning it through the hwmon
callback.

Fixes: 39671a1 ("hwmon: (isl28022) new driver for ISL28022 power monitor")
Cc: stable@vger.kernel.org
Signed-off-by: Sanman Pradhan <psanman@juniper.net>
Link: https://lore.kernel.org/r/20260410002613.424557-1-sanman.pradhan@hpe.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: Sanman Pradhan

drivers/hwmon/isl28022.c

@@ -9,6 +9,7 @@
 #include <linux/err.h>
 #include <linux/hwmon.h>
 #include <linux/i2c.h>
+#include <linux/math64.h>
 #include <linux/module.h>
 #include <linux/regmap.h>
 
@@ -185,8 +186,8 @@ static int isl28022_read_power(struct device *dev, u32 attr, long *val)
 				  ISL28022_REG_POWER, &regval);
 		if (err < 0)
 			return err;
-		*val = ((51200000L * ((long)data->gain)) /
-			(long)data->shunt) * (long)regval;
+		*val = min(div_u64(51200000ULL * data->gain * regval,
+				   data->shunt), LONG_MAX);
 		break;
 	default:
 		return -EOPNOTSUPP;