iio: adc: qcom-spmi-vadc: use scoped device_for_each_child_node()

iio: adc: qcom-spmi-vadc: use scoped device_for_each_child_node()

Switch to device_for_each_child_node_scoped() to simplify the code by
removing the need for calls to fwnode_handle_put() in the error path.

This prevents possible memory leaks if new error paths are added without
the required call to fwnode_handle_put().

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20240926-iio_device_for_each_child_node_scoped-v1-2-64ca8a424578@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>