MINOR: mux-quic: adapt return value of qcc_decode_qcs()

MINOR: mux-quic: adapt return value of qcc_decode_qcs()

Change return value of qcc_decode_qcs(). It now directly returns the
value from app_ops rcv_buf callback. Function documentation is updated
to reflect this.

For now, qcc_decode_qcs() return value is ignored by callers, so this
patch should not have any functional change. However, it will become
necessary when implementing multiple Rx buffers per QCS, as a loop will
be implemented to invoke qcc_decode_qcs() on several contiguous buffers.
Decoding must be stopped however as soon as an error is returned by
rcv_buf callback. This is also the case in case of a null value, which
indicates there is not enough data to continue decoding.