ASoC: SPEAr: remove custom DMA alloc compat function

spear_pcm_request_chan() is almost identical to
dmaengine_pcm_compat_request_channel(), with the exception that the
latter:

a) Assumes that the DAI DMA data is a struct snd_dmaengine_dai_dma_data
   pointer rather than some custom type.

b) dma_data->filter_data rather than dma_data should be passed to
   snd_dmaengine_pcm_request_channel() as the filter data.

Make minor changes to the SPEAr DAI drivers so that those two conditions
are met. This allows removal of the custom .compat_request_channel().

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>