greybus: define greybus_data_sent()

Define greybus_data_sent(), which is a callback the host driver
makes when a buffer send request has completed.  The main use for
this is to actively detect errors that can occur while sending.
(Something like this existed at one time and was removed.)

This also defines gb_hd_message_find(), which looks up a message
pointer associated with a buffer sent over a given host device.
This is now a pretty trival mapping.

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