ALSA: hda/realtek - Fix the wrong offset for two-speaker systems

When the machine has two speakers but wants to put more multi-io
jacks, the parser shouldn't consider about the shared DAC but try to
assign the individual DACs.  Otherwise the channel mapping would be
fairly confused and lead to the wrong DACs.

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