diag: dci: Check if the DCI client is valid before signaling

Diag driver signals the DCI clients whenever there is a change in
the DCI channel status. This patch adds a check to see if the client
is valid before signaling.

Change-Id: I5b2be3a56494b259d76979167d1aae034c4b277a
CRs-Fixed: 535604
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>