pad: Ensure last flow return is set on sink pads in push mode

The last flow return field was never updated on sink pads in push mode. This
fixes it and makes it consistent.

Part-of: <gstreamer/gstreamer!870>
39 jobs for !870 with backport-mr-868-into-1.18-lastflowret in 57 minutes and 24 seconds (queued for 12 seconds)
merge request