ASoC: rockchip: i2s: fix maxburst of dma data to 4

Since RK3288 DMAC's burst length only support max to 4, here
set maxburst of playback and capture dma data to 4.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>