[ALSA] hda-codec - Change Gigabyte K8N51 from 6stack to 6stack-digout

This patch moves the entry for the Gigabyte K8N51 from the 6stack
grouping to the 6stack-digout grouping, allowing for S/PDIF output
functionality.

Signed-off-by: Tobin Davis <tdavis@dsl-only.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>