ASoC: hdac_hdmi: Fix to check num nodes correctly

commit 3c83ac23 ("ASoC: hdac_hdmi: check error return") fixes
the static checker warning reported by Dan Carpenter:

	sound/soc/codecs/hdac_hdmi.c:416 hdac_hdmi_parse_and_map_nid()
	warn: unsigned 'hdac->num_nodes' is never less than zero.

But it doesn't fix the issue completely.

It's also a failure if no sub nodes found for an afg node. So modify
the return condition appropriately.

Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>