msm: sps: update the checking of free slots

There is a new BAM use case where the client driver does not use
either interrupt or polling for a pipe. Thus, add a new way to
check the free slots in descriptor FIFO for this case.

Change-Id: I133b938718d0c3654309e65ac0670fe1725a937f
Signed-off-by: Yan He <yanhe@codeaurora.org>