ASoC: ux500: remove custom DMA alloc compat function

FIXME: Lee Jones is sending this patch; I'm just assuming it's been
applied already...

Signed-off-by: Stephen Warren <swarren@nvidia.com>
ASoC: dmaengine: remove compat_request_channel

struct snd_dmaengine_pcm_config's compat_request_channel field is no
longer set anywhere. Remove it, and the code that implements it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>