iio: adc: xilinx-ams: use device_* to iterate over device child nodes

Use `device_for_each_child_node_scoped()` in `ams_parse_firmware()`
to explicitly state device child node access, and simplify the child
node handling as it is not required outside the loop.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Link: https://patch.msgid.link/20240820-device_child_node_access-v3-1-1ee09bdedb9e@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>