ALSA: hda - Fix NULL dereference in snd_hda_gen_build_controls()

When no controls are assigned in the parser (e.g. no analog path),
spec->kctls.list is still NULL.  We need to check it before passing to
snd_hda_add_new_ctls().

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