ucm: return error if boot_list is empty
ucm: return error if boot_list is empty

If bootlist is empty, that means there is no BootSequence defined
in the ucm for this sound card, let set_boot_user() return error,
then alsactl could have chance to use non-ucm ways to initialize the
sound card.

BugLink: https://github.com/alsa-project/alsa-lib/pull/94
Signed-off-by: Hui Wang <51909cde9c974b33f1f2fb15bb7fd44a1b667281@canonical.com>
Signed-off-by: Jaroslav Kysela <72edd5e1a572f82f059ccbb83c92b74d6cb051ff@perex.cz>

alsa-project/alsa-lib,tiwai/alsa-lib,tiwai/alsa-lib,tiwai/alsa-lib,tiwai/alsa-lib,alsa-project/alsa-lib,alsa-project/alsa-lib,alsa-project/alsa-lib