net: mvpp2: 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.

The child nodes within mvpp2_probe are not accessed outside the loops,
and the scoped version of the macro can be used to automatically
decrement the refcount on early exits.

Use `device_for_each_child_node()` and its scoped variant to indicate
device's direct child nodes.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>