ASoC: samsung: Specify DMA channels through struct snd_dmaengine_pcm_config

The DMA channel names are specified through struct snd_dmaengine_pcm_config
rather than using SND_DMAENGINE_PCM_FLAG_CUSTOM_CHANNEL_NAME flag when
booting with devicetree in order to properly support deferred probing.
Without this change the sound machine driver initialization can complete
successfully with unavailable DMA resources.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>