diag: Protect wakeup source reference counts

Diag driver should protect the wakeup source reference counts when
writing data to USB and when USB writes are complete. This patch
protects the counters in these two contexts.

Change-Id: I6df26cbf8b65e2a87bd3043e3449ddd9877f8630
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>