diag: dci: Add mutex protection while accessing client details

Currently while extracting events and logs information from
the data read over peripherals, the clients details are accessed
without mutex protection. As the client access may happen from
multiple context, mutex protection is needed. This patch
resolves the same.

Change-Id: I9bd115e1cd9eebc625f4a68854d554ff874d866d
Signed-off-by: Katish Paran <kparan@codeaurora.org>