ALSA: hda: SPDIF mux name change

Changed the mux naming scheme from "IEC9258 Mux" to "IEC958 Playback Source" to match
the coding style.

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