use snd_dmaengine_pcm_prepare_slave_config to set slave config,
and remove the max_burst_size = 4 hard code.

select SND_SOC_GENERIC_DMAENGINE_PCM for mmp-pcm.
Signed-off-by: Qiao Zhou <zhouqiao@marvell.com>
Signed-off-by: Mark Brown <broonie@linaro.org>