ALSA: hda - Fix double-headphone/speaker paths for Cxt auto-parser

When multiple headphones or speakers are assigned but no individual
DACs are available, the driver should take the first HP/SPK DAC instead
of another primary output.  The patch adds a bit-flag to dac field of
struct pin_dac_pair indicating that it's a slave DAC.

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