ASoC: soc-pcm: use dai_get_widget() at dpcm_end_walk_at_be()

dpcm_end_walk_at_be() has very duplicate code.

	dpcm_end_walk_at_be() {
		...
		if (stream == SNDRV_PCM_STREAM_PLAYBACK) {
(1)			/* code for Playback */
		} else  {
(2)			/* code for Capture */
		}
	}

The difference between Playback (1) and Capture (2) code is pointer only
(= "playback_widget" or "caputre_widget").
OTOH, now we already has dai_get_widget() for it.
This means we can merge (1) and (2).
This patch do it and remove duplicated code.

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