ALSA: hda: SPDIF mux fixes for STAC927x

Corrected bounds-checking in stac92xx_auto_create_mux_input_ctls() and added a spec->spdif_labels
pointer for custom SPDIF mux labels for non-standard codec connections.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>