CLEANUP: quic: Rely on the packet length set by qc_lstnr_pkt_rcv()

CLEANUP: quic: Rely on the packet length set by qc_lstnr_pkt_rcv()

This function is used to parse the QUIC packets carried by a UDP datagram.
When a correct packet could be found, the ->len RX packet structure value
is set to the packet length value. On the contrary, it is set to the remaining
number of bytes in the UDP datagram if no correct QUIC packet could be found.
So, there is no need to make this function return a status value. It allows
the caller to parse any QUIC packet carried by a UDP datagram without this.