iio: adc: stm32: 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.

Note this would have made the bug fixed in the previous path much
less likely as it allows for direct returns.

Took advantage of dev_err_probe() to futher simplify things given no
longer a need for the goto err.

Cc: Olivier Moysan <olivier.moysan@foss.st.com>
Tested-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20240330185305.1319844-5-jic23@kernel.org


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