ALSA: ppc: Switch to use for_each_child_of_node() macro

Use for_each_child_of_node() macro instead of open coding it.
No functional change.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220916141108.683080-1-yangyingliang@huawei.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>