ASoC: soc-dpcm: tidyup for_each_dpcm_xx() macro

ASoC: soc-dpcm: tidyup for_each_dpcm_xx() macro

for_each_dpcm_xx() macro is using "dpcm" as parameter (1),
but, it is also struct member (2).

	#define for_each_dpcm_fe(be, stream, dpcm) \
	list_for_each_entry(dpcm, &(be)->dpcm[stream]...)
	                    ^^^^(1)      ^^^^(2)

Thus, it will be compile error if user not used "dpcm" as parameter

	for_each_dpcm_fe(be, stream, dp)
	                             ^^
This patch fixup it.

Reported-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87tv7x7idx.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>