ASoC: imx-pcm-dma: simplify pcm_config

The generic snd_dmaengine_pcm is able to retrieve all the needed
information from the attached dmaengine and is in fact able to
provide much more accurate flags to userspace, like the SDMA engine
being only able to operate in batch mode.

To avoid any future inconsistencies between the dmaengine and the
pcm_config, rip out the fixed config and rely on the core to fill
in the right flags derived from the dmaengine information.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>