diag: Fix incorrect handling of buffers

In diag driver, processing dci transaction takes place even if the
incoming buffer is invalid. Also, we are handling the memory pools
incorrectly. This patch fixes these two issues

Change-Id: Ibdcc9759e3e1030f34f82c4e6b0315a8e5c79ab7
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>