diag: Mutex protect the command list while traversing

During peripheral subsystem restart apps diag remove the entries
registered for the peripheral. In the meantime, tool can send
command designated to that peripheral. Mutex protect the list
so that stale data is not accessed.

Change-Id: Icca4b43506544b54e516110fbed97c8f39602861
Signed-off-by: Katish Paran <kparan@codeaurora.org>