ALSA: HDA VIA: Fix 2nd S/PDIF out function

ALSA: HDA VIA: Fix 2nd S/PDIF out function

As it seems, the recently-sent patch for the 2nd S/PDIF (HDMI) output
is not working with alsa-kernel 1.0.18rc3.

This patch makes it work by
* activating the second S/PDIF output pin in the pin config
* consolidating the dig_playback_pcm_prepare() with extra_dig_pcm_prepare()
  functions
* remove the need for an extra hda_pcm_stream structure and rather represents
  the second digital output as substream within the primary S/PDIF digital out
  stream.

Signed-off-by: Logan Li <[email protected]>
Signed-off-by: Harald Welte <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>
Signed-off-by: Jaroslav Kysela <[email protected]>