Fixes for ISP/SAF1761 host mode:
- Make the USB hardware skip PTDs which are not allocated.
- Peek host memory twice. Sometimes the PTD status is incorrectly
returned as zero.
- Ensure the host channel is always freed when software TD
is completing.
- Add correct configuration of interrupt polarity and type.
- Set CERR to 2 for asynchronous traffic to avoid having to
reactivate the PTD when a NAK token is received.
- Fix detection of STALL PID.

Sponsored by:	DARPA, AFRL