ASoC: soc-core: tidyup for snd_soc_dapm_add_routes()

snd_soc_dapm_add_routes() registers routes by using
for(... i < num; ...). If routes was NULL, num should be zero.
Thus, we don't need to check about route pointer.
This patch also cares missing return value.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87d0hhahon.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>