platform-drivers: msm: sps: return number of unused descriptors

Client driver has a new feature requirement to acquire the number
of unused descriptors in the descriptor FIFO of a pipe; Add this
support in SPS driver.

Change-Id: Idf02308f6011973c2ad4fea2a50134b48062b953
Signed-off-by: Yan He <yanhe@codeaurora.org>
4 files changed