greybus: loopback: add gb_loopback_operation_sync

In order to extract timestamps from gb_message instead of gb_connection we
will need access to the gb_operation structure. A first step to that is to
create our own gb_loopback_operation_sync which will call
gb_operation_request_send_sync() directly. Once loopback is using this
function internally it will be possible to convert to gb_message based
timestamps and drop gb_connection based timestamps in two seperate patches.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>