ALSA: hda/realtek - Fix DAC assignments of multiple speakers

When a device has multiple speakers and still has the auto-mute support,
the driver copies line_outs[] to speaker_outs[].  And then it tries to
assign DACs for both.  This ended up with the assignment only to the
primary DAC to all speakers.

This patch fixes the situation by checking the duplicated LO/SPK case
appropriately.

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