diag: dci: Protect the client list and command entries

Currently, on open and close notification for channels,
the dci clients are updated accordingly. The list
traversal for active dci clients are not mutex protected.
Also searching for command registration in table is not
properly protected. This patch mutex protect the list
traversal and command registration table search to avoid
stale entries.

Change-Id: I015d6674afd605c63cd01ceec0109d9da5462629
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>