greybus: get rid of message status

We (sort of) maintain the status of each message, but we shouldn't
need to.  Right now we're not using it consistently in any case.

If a message fails to send, the caller will know to destroy the
operation that contained it.

If a message has been sent (i.e., handed to the host device layer)
it'll have a non-null cookie pointer.

If a does complete in error, we can update the status of the
operation that contains it.  That isn't happening right now but
it will soon.

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