hwmon: (isl28022) Fix current reading calculation

According to the ISL28022 datasheet, bit15 of the current register is
representing -32768. Fix the calculation to properly handle this bit,
ensuring correct measurements for negative values.

Signed-off-by: Yikai Tsai <yikai.tsai.wiwynn@gmail.com>
Link: https://lore.kernel.org/r/20250519084055.3787-2-yikai.tsai.wiwynn@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: yikaitsaiww

drivers/hwmon/isl28022.c

@@ -154,15 +154,17 @@ static int isl28022_read_current(struct device *dev, u32 attr, long *val)
 	struct isl28022_data *data = dev_get_drvdata(dev);
 	unsigned int regval;
 	int err;
+	u16 sign_bit;
 
 	switch (attr) {
 	case hwmon_curr_input:
 		err = regmap_read(data->regmap,
 				  ISL28022_REG_CURRENT, &regval);
 		if (err < 0)
 			return err;
-		*val = ((long)regval * 1250L * (long)data->gain) /
-			(long)data->shunt;
+		sign_bit = (regval >> 15) & 0x01;
+		*val = (((long)(((u16)regval) & 0x7FFF) - (sign_bit * 32768)) *
+			1250L * (long)data->gain) / (long)data->shunt;
 		break;
 	default:
 		return -EOPNOTSUPP;