iio: adc: rcar-gyroadc: use for_each_available_child_node_scoped()

Using automated cleanup to replace of_node_put() handling allows for
a simplfied flow by enabling direct returns on errors.

Non available child nodes should never have been considered; that
is ones where status != okay and was defined.

Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240225142714.286440-5-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>