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 <hui.wang@canonical.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>