diag: dci: Protect dci client's data buffer with mutex

Data coming from smd is processed and written into client's data
buffer. The read thread reads the data from the buffer and resets
the length of the data in the buffer. This change fixes the
concurrency problem.

Change-Id: I52b8ed8c930ff0f0a978e9ccc3a34479ad253f0b
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>