diag: Send error response for un-supported commands

Currently, diag is not sending any response for
un-supported commands due to which time-out is
happening and tool is re-sending those commands.
The patch fixes issue by sending error response
for those commands.

Change-Id: I07247ad003203070b227c988e8a09a6c3bd12994
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>