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

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: 5691b23489db ("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>