greybus: add gb_operation_find()

greybus: add gb_operation_find()

Add a red-black tree indexed by operation id to a connection to
allow pending operations (whose requests are in-flight) to be
found when their matching response is recieved.

Assign the id at the time an operation is inserted, and update
the operation's message header(s) to include it.

Rename gb_connection_op_id() to be more consistent with the
naming conventions being used elsewhere.

(Noting now that this may switch to a simple list implementation
based on Greg's assertion that lists are faster than red-black trees
for up to a few hundred entries.)

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