iio: adc: ad7768-1: use device_* to iterate over device child nodes

iio: adc: ad7768-1: use device_* to iterate over device child nodes

Drop the manual access to the fwnode of the device to iterate over its
child nodes. `device_for_each_child_node` macro provides direct access
to the child nodes, and given that they are only required within the
loop, the scoped variant of the macro can be used.

Use the `device_for_each_child_node_scoped` macro to iterate over the
direct child nodes of the device.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240801-device_child_node_access-v1-2-ddfa21bef6f2@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>