diag: Reallocate dci buffer with proper required capacity

DCI command buffer reallocation is not done properly with
required capacity. The patch reallocates the same buffer and
updated capacity properly with header and response length.

Change-Id: I7b5fd132b9241d0f1493bcb602a6b361e4ad9a04
Signed-off-by: Hardik Arya <harya@codeaurora.org>