ASoC: samsung: pass filter function as pointer

As we are now passing the filter data as pointers to the drivers,
we can take the final step and also pass the filter function the
same way. I'm keeping this change separate, as there it's less
obvious that this is a net win.

Upsides of this are:

- The ASoC drivers are completely independent from the DMA engine
  implementation, which simplifies the Kconfig logic and in theory
  allows the same sound drivers to be built in a kernel that supports
  different kinds of dmaengine drivers.

- Consistency with other subsystems and drivers

On the other hand, we have a few downsides:

- The s3c24xx-dma driver now needs to be built-in for the ac97 platform
  device to be instantiated on s3c2440.

- samsung_dmaengine_pcm_config cannot be marked 'const' any more
  because the filter function pointer needs to be set at runtime.
  This is safe as long we don't have multiple different DMA engines
  in thet same system at runtime, but is nonetheless ugly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>