rtppassthrough: fix rtp-stats message compatibility with GstRTPBasePayload

rtppassthrough: fix rtp-stats message compatibility with GstRTPBasePayload

"clock-rate" and "pt" are G_TYPE_UINT in the base class, so let's
keep them like that here too, since the entire purposes of the
passthrough element is to fake being a payloader. The types in the
message don't have to be consistent with the types in the caps.

Reverts part of commit a6fa53b7 of !7526

https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7552#note_2576653

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7784>