BUG/MINOR: mux-quic: fix show quic report of QCS prepared bytes

BUG/MINOR: mux-quic: fix show quic report of QCS prepared bytes

On show quic, each MUX streams are listed with their various indicator
for buffering on Rx and Tx. In particular, txoff displays in parenthesis
the current level of data prepared by the upper stream instance not yet
emitted by QUIC transport layer.

This value is only accessible after a substract operation. However,
there was a typo which caused the result to be always 0. Fix this by
reusing the correct offsets in the calculation.

This should be backported up to 3.0.