diag: Protect shared resources while mask updates

DIAG sends mask updates to peripherals over SMD control channels.
It uses several buffers, variables and SMD channel for this update
process. These shared resources risk corruption from several work
queue functions. Putting them under a mutex will ensure they do
not get overwritten.

Change-Id: I1d0a82b60dd8384d4b44ace29b25d60afe2ac5ad
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>