diag: Add message support for max packet size cmd/rsp messages

Tools that run on the PC need to know the maximum packet size
for cmd/rsp messages so they can appropriately allocate
memory and if the payload is large enough, break the message
up into multiple calls.  Add support for returning the
maximum packet size for cmd/rsp messages.

Change-Id: Id53eaef6d6c67f7a22ed8d2ae4b82e2aa6794e65
Signed-off-by: Dixon Peterson <dixonp@codeaurora.org>