hwmon: (f71882fg) prevent possible division by zero

In the unlikely event that something goes wrong with the device and
its registers, the fan_from_reg() function may return 0. This value
will cause a division-by-zero error in the show_pwm() function.

To prevent this, test the value of
fan_from_reg(data->fan_full_speed[nr]) against 0 before performing
the division. If the division-by-zero error is avoided, assign 0 to
the val variable.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: df9ec2da ("hwmon: (f71882fg) Reorder symbols to get rid of a few forward declarations")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Link: https://lore.kernel.org/r/20230510143537.145060-1-n.zhandarovich@fintech.ruSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/f71882fg.c

@@ -1096,8 +1096,11 @@ static ssize_t show_pwm(struct device *dev,
 		val = data->pwm[nr];
 	else {
 		/* RPM mode */
-		val = 255 * fan_from_reg(data->fan_target[nr])
-			/ fan_from_reg(data->fan_full_speed[nr]);
+		if (fan_from_reg(data->fan_full_speed[nr]))
+			val = 255 * fan_from_reg(data->fan_target[nr])
+				/ fan_from_reg(data->fan_full_speed[nr]);
+		else
+			val = 0;
 	}
 	mutex_unlock(&data->update_lock);
 	return sprintf(buf, "%d\n", val);