[ALSA] hda-codec - Allow multiple SPDIF devices

The current code doesn't allow multiple SPDIF devices, and causes
errors when multiple SPDIF devices are found (e.g. SPDIF out and HDMI).
This patch allows multiple SPDIF devices by incrementing the index
automatically.

Signed-off-by: Takashi Iwai <tiwai@suse.de>