ALSA: HDA: hda_local: Less magic numbers.

Explain some of the magic numbers I saw while trying to fix the AD1989 SPDIF
issue. Maybe should just use the expanded form directly in the verbs?

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>