ASoC: Drop SND_DMAENGINE_PCM_FLAG_CUSTOM_CHANNEL_NAME flag

Since commit 194c7dea
"ASoC: dmaengine: add custom DMA config to snd_dmaengine_pcm_config"
custom DMA channels can be also specified in chan_names[] field of
struct snd_dmaengine_pcm_config.  This patch removes chan_name field
of struct snd_dmaengine_dai_dma_data as it is now unused.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>