greybus: record gbuf->operation

greybus: record gbuf->operation

Currently a gbuf records a pointer to the connection it's associated
with.  We now know only use gbufs in operation messages, so we can
point a gbuf at its operation instead.  This still gives access to
the connection where needed, but it also will provide all the
context we'll ever need for a gbuf, and this allows us (in the next
patch) to remove the gbuf->context field as well.

So switch to recording in a gbuf the operation rather than the
connection it is associated with.

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