ALSA: hda - Fix secondary SPDIF on VT1708S and VT1702 codecs

VIA VT1708S and VT1702 codecs can have two SPDIF outputs.  One of them
should have been handled as the extra digital out, but it's not
properly accessed.

This patch fixes the handling of the secondary SPDIF on these codecs
with the slave dig-out as found in patch_sigmatel.c.  This makes the
use of such a device easier (for normal users).

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