ALSA: intel8x0 - add Dell Optiplex GX620 (AD1981B) to AC97 clock whitelist

alsa-info.sh output at:
https://bugzilla.redhat.com/show_bug.cgi?id=441087#c49

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>