[ALSA] via82xx: add support for VIA VT8251 (AC'97)

Add support for VIA VT8251 AC'97. Includes a workaround which ensures sound
won't stop playing after one second of playback.

Signed-off-by: Bastiaan Jacques <b.jacques@planet.nl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>