greybus: rename message buffer fields

The beginning of an operation message always contains the message
header.  Rename the "buffer" field in an operation message to
be "header" to reflect this.  Change its type as well.

The size of a message is the combined size of its header and its
payload.  Rename the "buffer_size" field in a message header to
be simply "size", so message->size describes exactly that.

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