ASoC: fsi: reserve prefetch period on DMA transferring

Current FSI is supporting DMAEngine transfer,
but, it needs to use work queue.
Therefore, DMA transfer settings might be late if there is heavy task.
This patch reserves next period beforehand on DMA transfer function.
Android sound will be breaking up without this patch.

Tested-by: Tomohito Esaki <etom@igel.co.jp>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>