[ALSA] ac97 - Fix silent output problem with Cx20551 codec

Fixed the silent output problem on laptops with Conexant Cx20551 codec
chip, such as Packard-bell EasyNote A* series.
The information was taken from ALSA bug#1134.

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