ALSA: hda/realtek - Fix a wrong condition

sparse complains that "spec->multiout.dac_nids" is a pointer.

sound/pci/hda/patch_realtek.c:2321:37: error: incompatible types for operation (>)
sound/pci/hda/patch_realtek.c:2321:37:    left side has type unsigned short const [usertype] *dac_nids
sound/pci/hda/patch_realtek.c:2321:37:    right side has type int

It was meant to be num_dacs instead of dac_nids.
Although the current code still works as expected (when num_dacs is zero,
dac_nids should be NULL, too), better to fix now, of course.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>