greybus: tracing: fix message traces

The original message trace events were defined long before the
recent tracing updates.  It records information that's not
really directly related to a message.  Change the information
recorded and reported for message events to just be the content
of the message header.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>