ALSA: hda - Fix no NID error with VIA codecs

The via driver spews warnigs like
	hda-codec: no NID for mapping control Independent HP:0:0
with some codecs because snd_hda_add_nid() is called with nid=0.
This patch fixes it by skipping the call when no corresponding widget
is found.

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