ASoC: edma-pcm: Add empty functions for !CONFIG_SND_EDMA_SOC builds

So drivers mixing with other platform drivers, like davinci-mcasp do not
need to fiddle with CONFIG_SND_EDMA_SOC in their code.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Mark Brown <broonie@linaro.org>