ALSA: HDA: patch_analog: Implement multiple outputs for AD1988

Implement the slave_dig_outs for AD1989 cards. Untested due to lack of
hardware.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>