ASoC: remove dpcm_xxx flags

Merge series from Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>:

Now, we don't need to use dpcm_playback/capture flags for DPCM since v6.11,
and thus we can use playback/capture_only flags instead too for DPCM,
which is same as non-DPCM.

Let's remove dpcm_playback/capture flags, and use playback/capture_only flags
instead.

[Before]			[After]
	dpcm_playback = 1;	/* no setting is needed */
	dpcm_capture  = 1;

	dpcm_playback = 1;	playback_only = 1;
	dpcm_capture  = 0;

	dpcm_playback = 0;	capture_only = 1;
	dpcm_capture  = 1;

And then, because no-one uses dpcm_xxx flags, we don't need to have the code
for both DPCM and non-DPCM. These can be handled by same code.