sending streamer frames when link is down causes weird behavior:
- the frames are counted as sent, while they are not
- some frames are dropped, depending on PHY implementation
- at startup, the link is reseted by software, so link is up for
  some time, then it goes down, than up again... mess

two additions:
- the dreq_o signal is gated with link_ok_i, so that frames cannot
  be sent when link is down
- startup counter which delays the start of sending frames