MEDIUM: mux: Remove const on the buffer in mux->snd_buf()

MEDIUM: mux: Remove const on the buffer in mux->snd_buf()

This is a partial revert of the commit deccd1116 ("MEDIUM: mux: make
mux->snd_buf() take the byte count in argument"). It is a requirement to do
zero-copy transfers. This will be mandatory when the TX buffer of the
conn_stream will be used.

So, now, data are consumed by mux->snd_buf() and not only sent. So it needs to
update the buffer state. On its side, the caller must be aware the buffer can be
replaced y an empty or unallocated one.

As a side effet of this change, the function co_set_data() is now only responsible
to update the channel set, by update ->output field.