ASoC: sprd: Fix modular build

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Suggested-by: Baolin Wang <baolin.wang@linaro.org>
Tested-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sprd/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # Spreadtrum Audio Support
 
-obj-$(CONFIG_SND_SOC_SPRD)	+= sprd-pcm-dma.o sprd-pcm-compress.o
+snd-soc-sprd-platform-objs := sprd-pcm-dma.o sprd-pcm-compress.o
+
+obj-$(CONFIG_SND_SOC_SPRD) += snd-soc-sprd-platform.o