ALSA: hda - Fix DAC filling for multi-connection pins in Realtek parser

Fix a regression in the DAC filling code in patch_realtek.c.  The already
filled DACs in multiout.dac_nids[] were ignored because of num_dacs=0,
thus always pointed to the first DAC.

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