diag: Handle Mutexes correctly in the case of Callback data

Diag driver grabs a mutex incorrectly when processing data from the
callback process leading to a deadlock. This patch fixes this
problem.

Change-Id: I759d702b5da93661f3f75a95c176e5dc59d46eed
CRs-Fixed: 565082
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>