ALSA: pcm: Fix referred substream in snd_pcm_action_group() unlock loop

In the unlock loop of snd_pcm_action_group(), the object "s" is used
as the check of nonatomic PCM, but it should be rather "s1", which is
the iterator of the loop.  This supposedly causes a kernel panic when
the substreams in operatino are linked.

Fixes: 257f8cce

 ('ALSA: pcm: Allow nonatomic trigger operations')
Reported-and-tested-by: Arthur Marsh <arthur.marsh@internode.on.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>