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 (merged)