Some further DWC OTG improvements for full speed and low speed devices:
- Revert r265427. It appears we are halting the DWC OTG host
controller schedule if we process events only at every SOF. When doing
split transactions we rely on that events are processed quickly and
waiting too long might cause data loss.
- We are not always able to meet the timing requirements of interrupt
endpoint split transactions. Switch from INTERRUPT to CONTROL endpoint
type for interrupt endpoint events until further, hence CONTROL
endpoint events are more relaxed, reducing the chance of data
loss. See comment in code for more in-depth explanation.
- Simplify TT scheduling.

MFC after:	3 days