diag: dci: Handle incoming dci buffers correctly

The incoming dci packets contain the length of the response, log or
event expected. This patch adds checks to the size of the packet received
to be greater or equal to the length mentioned in the packet.

Change-Id: I308d3fc369c6117dd8ca024e60e044f0047bb679
CRs-Fixed: 530487
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>