ASoC: rt5682: use devm_snd_soc_register_component()

We have devm_xxx version of snd_soc_register_component,
let's use it.

This patch also removes related empty functions

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>