ASoC: hdac_hdmi: check error return

As hdac->num_nodes is unsigned we can not check if
snd_hdac_get_sub_nodes() has returned error or success. Lets have a
temporary int to check the error value.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Mark Brown <broonie@kernel.org>