diag: Free global buffer properly after receiving write done

Currently diag apps buffer are not free properly after
receiving write done completion, which is leading to use
after free. The patch frees global hdlc and non_hdlc data
buffer from mempool properly after receiving write done.

Change-Id: If60de6be206ddd796a876dcf8060259cfd8a5424
Signed-off-by: Hardik Arya <harya@codeaurora.org>