leds: pca995x: Use device_for_each_child_node() to access device child nodes

leds: pca995x: Use device_for_each_child_node() to access device child nodes

The iterated nodes are direct children of the device node, and the
`device_for_each_child_node()` macro accounts for child node
availability.

`fwnode_for_each_available_child_node()` is meant to access the child
nodes of an fwnode, and therefore not direct child nodes of the device
node.

Use `device_for_each_child_node()` to indicate device's direct child
nodes.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240805-device_for_each_child_node-available-v3-2-48243a4aa5c0@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>