ALSA: hda - Fix mix->DAC deduction for ALC892

The current alc662 parser doesn't set the DAC for the mixer 0x0f
properly for ALC892, which has 4 DACs while ALC662 has 3.
Fixed by implementing alc662_mix_to_dac() more genericly with the
dynamic widget list.

Signed-off-by: Takashi Iwai <tiwai@suse.de>