webrtcbin: Fix transceiver matching with FEC decoder for PCMU
When using PCMU, the payload type can be 0. When rtpbin requests
FEC decoder and passes a payload type of 0, the check on payload
type in try_match_transceiver_with_fec_decoder resulted in the
FEC decoder not being set on the transceiver. This then resulted
in code setting pass through on the rtpulpfecdec being bypassed
in _set_internal_rtpbin_element_props_from_stream. We need to
set pass through on rtpulpfecdec as it is broken.