BUG/MEDIUM: stream-int: always clear CS_FL_WANT_ROOM before receiving

BUG/MEDIUM: stream-int: always clear CS_FL_WANT_ROOM before receiving

Commit d94f877cd ("BUG/MINOR: mux_pt: Set CS_FL_WANT_ROOM when count is
zero in rcv_buf() callback") triggered a pending issue with this flag,
which is that it's cleared too late and sometimes causes some Rx
transfers to stall. We need to clear it before attempting to receive
otherwise we may risk to see an earlier copy of the flag.

Note that it should probably be defined that this flag could be purged
on each invocation of mux->rcv_buf(), which would make sense.

No backport is needed.