diag: Send error response in case for bad packet

Currently, apps diag is not sending error response, in case
packet received from tool is raw and apps diag is in hdlc
enabled mode. Tool needs a response to trigger its recovery
algorithm. This patch ensures that error response is sent in
case for bad packet.

Change-Id: I8048698e215dd72cb4fe964232c942aaa0ae2e33
Signed-off-by: Katish Paran <kparan@codeaurora.org>