iio: ad9523: fix a missing check of return value

If ad9523_write() fails, indio_dev may get incorrect data. The fix
inserts a check for the return value of ad9523_write(), and it fails,
returns an error.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>