iio: dac: ad5421: use int type to store negative error codes

Change the 'ret' variable in ad5421_update_ctrl() from unsigned int to
int, as it needs to store either negative error codes or zero returned
by ad5421_write_unlocked().

Fixes: 5691b23 ("staging:iio:dac: Add AD5421 driver")
Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://patch.msgid.link/20250901135726.17601-3-rongqianfeng@vivo.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: qianfengrong

drivers/iio/dac/ad5421.c

@@ -186,7 +186,7 @@ static int ad5421_update_ctrl(struct iio_dev *indio_dev, unsigned int set,
 	unsigned int clr)
 {
 	struct ad5421_state *st = iio_priv(indio_dev);
-	unsigned int ret;
+	int ret;
 
 	mutex_lock(&st->lock);