greybus: pass messages to host drivers

Pass structured greybus messages rather than buffers to the host
drivers.

This will allow us to separate the transfer buffers from the message
structures.

Rename the related functions to reflect the new interface.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>