diag: Release wake source properly

Currently, diag driver is not releasing wake
source on receiving 0 length packet from SMD.
This patch ensures that the wake sources
hold are released properly.

Change-Id: I4eefc0d361a130c7f313b8ba215f51ecf32e0923
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>