diag: Do not overwrite the earlier responses

If hdlc is disabled on apps, the buffer length is not being
properly increased. Thus the overwrite is happening with the new
responses. This patch fixes the issue.

Change-Id: I1dcbd666bdaab70eabf72547acf77c056cf345b1
Signed-off-by: Katish Paran <kparan@codeaurora.org>