diag: Save the correct task pointer while registering dci client

During deregistration of dci client the task pointer is not
matching with the task pointer saved during registration.
Made a change to store the correct pointer during dci client
registration.

Change-Id: I94123d2043532c456ac4b62435e57dac71adafbe
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>