ASoC: soc.h: use defined number instead of direct number

snd_soc_pcm_runtime has dpcm for Playback/Capture, but it is defined
directly "2". It should use defined number.

	struct snd_soc_pcm_runtime {
		...
=>		struct snd_soc_dpcm_runtime dpcm[2];
		...
	}

This patch fixup it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87r106v9mv.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>