ALSA: ppc: Fix of-node refcount unbalance

We forgot to unreference the node when aborting from the loop of
for_each_child_of_node() in snd_pmac_tumbler_init().  This leads to
unbalanced node refcount.  Fix it by adding the missing of_node_put()
call.

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