ALSA: ac97: Fix of-node refcount unbalance

ac97_of_get_child_device() take the refcount of the node explicitly
via of_node_get(), but this leads to an unbalance.  The
for_each_child_of_node() loop itself takes the refcount for each
iteration node, hence you don't need to take the extra refcount
again.

Fixes: 2225a3e6

 ("ALSA: ac97: add codecs devicetree binding")
Reviewed-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>