msm: sps: support using cached write pointer for BAM-to-Sys mode

Support using the cached write pointer for a BAM-to-Sys mode BAM
pipe when check if the desc FIFO of that pipe is full.

Change-Id: Ie480ecca9eff76388c504665b496e01bbf3e009f
Signed-off-by: Yan He <yanhe@codeaurora.org>