diag: Add support for MSM8939 QPST string

MSM8939 is a new device. Add diag support for sending
appropriate model number for MSM8939 so that QPST
can display the appropriate string.

Change-Id: If25f790e330bef89fc332b9595833be7df30c719
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>