ASoC: Revert "Drop SND_DMAENGINE_PCM_FLAG_CUSTOM_CHANNEL_NAME flag"

This reverts commit c6644119 and
restores the ability to specify DMA channel names per DAI dma_data.
Unfortunately the functionality removed in the patch being reverted
cannot be entirely replaced by specifying DMA channel names in struct
snd_dmaengine_pcm_config as that does not cover devices with more than
2 DMA channels.

Together with patch "ASoC: Revert "samsung: Remove unneeded
initialization of chan_name"" this fixes broken sound on the s3c24xx
SoC platforms.

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