msm: sps: Fix race condition in SPS debugfs APIs

SPS debugfs APIs can be called concurrently which can result
in dangling pointer access. This change synchronizes access
to the SPS debugfs buffer.

Change-Id: I409b3f0618f760cb67eba47b43c81d166cdae4aa
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>