ASoC: ak4642: tidyup DAPM route for playback

It needs DAC -> Playback route instead of direct settings via
SND_SOC_DAPM_DAC. otherwise, it can't find correct path if
sound card used prefix name

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