greybus: embed gbufs into operation message structure

Embed the gbuf structures for operation messages into the message
structure rather than pointing to a dynamically allocated one.

Use a null gbuf->transfer_buffer pointer rather than a null gbuf
pointer to indicate an unused gbuf.

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