greybus: operation: clean up greybus_message_sent

Add connection variable to greybus_message_sent.

This will be put to more use by a follow-up up patch.

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>