greybus: connection: fix connection initialisation

Make sure connection is fully initialised before registering the
connection device and adding it to the host-device and bundle lists.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Tested-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>