diag: dci: Copy the complete response coming from peripheral

Currently, if multiple responses comes over peripheral and the
user space client haven't read the last response or in middle
of reading the last response there is a possibility of
overwriting the last response. This patch ensures that the
offset is increased accordingly if multiple responses are
appended to same buffer.

Change-Id: I145756776b8902ab0819515b4192eb18d26949df
CRs-Fixed: 683417
Signed-off-by: Katish Paran <kparan@codeaurora.org>