ASoC: soc-core: tidyup for snd_soc_dapm_new_controls()

snd_soc_dapm_new_controls() registers controls by using
for(... i < num; ...). It means if widget was NULL, num should be zero.
Thus, we don't need to check about widget pointer.
This patch also cares missing return value.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87ftmdahow.wl-kuninori.morimoto.gx@renesas.com
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>