diag: Enable diag over rpmsg communication for wcnss

Enable apps to wcnss diag communication over rpmsg. Changes were
made to copy the data from smd buffer into diag buffer in the
callback and queue a work to process the data in work queue and
release the diag buffer in work queue.

Change-Id: I369312ad8e24204577e8e8fe1097542bf6ae7679
Signed-off-by: Sridhar Arra <sarra@codeaurora.org>