net: ethernet: ibm: Simpify code with for_each_child_of_node()

for_each_child_of_node can help to iterate through the device_node,
and we don't need to use while loop. No functional change with this
conversion.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240816015837.109627-1-zhangzekun11@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>