ASoC: msm: qdsp6v2: make audio debugfs read and release exclusive

A thread can read audio debugfs entry while another closes the
device. Protect these operations with a mutex and before read
check audio data to be a valid pointer.

Change-Id: If29a308c1a8329d7befd047d41abe5f6ab626199
Signed-off-by: Divya Ojha <dojha@codeaurora.org>