ALSA: hda - Fix a complile warning in patch_via.c

  sound/pci/hda/patch_via.c:2087: warning: 'dac' may be used uninitialized in this function

Signed-off-by: Wang Shaoyan <wangshaoyan.pt@taobao.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>