ASoC: lpass-platform: use dma_ch instead of rdma_ch/wrdma_ch

This patch cleans up usage of wrdma_ch and rdma_ch variables into a
common variable dma_ch, As there is no real use of tracking the dma
channel in two different variables based on stream direction.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Acked-by: Kenneth Westfield <kwestfie@codeaurora.org>