ASoC: dmaengine: Correct Makefile when sound is built as module

soc-dmaengine-pcm library need to be part of the snd-soc-core in order to
be able to compile ASoC as modules when dmaengine is enabled on the platform.
Signed-off-by: NPeter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/Makefile

 snd-soc-core-objs := soc-core.o soc-dapm.o soc-jack.o soc-cache.o soc-utils.o
 snd-soc-core-objs += soc-pcm.o soc-compress.o soc-io.o
 
-snd-soc-dmaengine-pcm-objs := soc-dmaengine-pcm.o
-obj-$(CONFIG_SND_SOC_DMAENGINE_PCM) += snd-soc-dmaengine-pcm.o
+ifneq ($(CONFIG_SND_SOC_DMAENGINE_PCM),)
+snd-soc-core-objs += soc-dmaengine-pcm.o
+endif
 
 obj-$(CONFIG_SND_SOC)	+= snd-soc-core.o
 obj-$(CONFIG_SND_SOC)	+= codecs/