diag: Add mutex protection while writing to smd

Add mutex protection to diag ctrl channel to avoid concurrency
issues with smd_write() to the diag ctrl channel.

Change-Id: Ief0867584da558a7952e68757da324430af3c58a
CRs-Fixed: 587775
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
6 files changed