hotfix: RLC AM reported wrong number of bytes to retransmit

This was causing oaisim to fail way too often at the startup
of a connection.

Things should go better with this patch.

It may not be the end of the story, the use of retrans_num_bytes_to_retransmit
is not clear to me. The problem I see is: what if the max number of
retransimission has been reached (if that has a meaning at all in RLC AM)?
will this value be decreased or not?