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: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>