iio: dac: ltc2688: Use device_for_each_child_node_scoped()

iio: dac: ltc2688: Use device_for_each_child_node_scoped()

Switching to the _scoped() version removes the need for manual
calling of fwnode_handle_put() in the paths where the code
exits the loop early. In this case that's all in error paths.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240217164249.921878-16-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>