ALSA: pcm: Don't call sync_stop if it hasn't been stopped

ALSA: pcm: Don't call sync_stop if it hasn't been stopped

The PCM stop operation sets the stop_operating flag for indicating the
sync_stop post-process.  This flag is, however, set unconditionally
even if the PCM trigger weren't issued.  This may lead to
inconsistency in the driver side.

Correct the code to set stop_operating flag only after the trigger
STOP is actually called.

Fixes: 1e850beea278 ("ALSA: pcm: Add the support for sync-stop operation")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210206203656.15959-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>