[ALSA] remove incorrect usage of SNDRV_PCM_INFO_SYNC_START and snd_pcm_set_sync()

Set the SNDRV_PCM_INFO_SYNC_START flag and the substream's sync ID
(only) if the substream actually can be linked to another one.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>