ASoC: rockchip: pdm: adjust waterlevel in frame unit

This patch make the waterlevel more reasonable, because the pdm
controller share the single FIFO(128 entries) with each channel.
adjust waterlevel in frame to meet the vad or dma frames request.

Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>