diag: Prevent out-of-bound access while processing dci transaction

Proper buffer length check is missing for dci userspace data
buffer before processing the dci transaction. The patch adds
proper check for the same.

Change-Id: I68c0e8c41d4e05493adecf8a1fcacea708dfafa2
Signed-off-by: Hardik Arya <harya@codeaurora.org>