greybus: start using struct gb_message

This converts some of the operation code to start leveraging the
new gb_message type.  Instead of creating the request and response
gbufs, we initialize (and tear down with a new function) the
request and response message structures.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>