diag: Fix possible double free issue

Currently, on receiving the write completion
notification from USB, there is possibility of
double freeing the buffer. This patch takes care
of this issue.

CRs-Fixed: 967601
Change-Id: I1207e0c3f103fa9dec6d766621aec8ccea62be69
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>