Given the discussions of last week, this removes urb->next from
the USB core API.  This change simplifies the driver API by getting
rid of a superfluous feature (and related new-developer confusion),
gets rid of a hidden failure mode (drivers can now see resubmit
failures), lets us get rid of a HCD feature that isn't consistently
implemented, and so on.

This will break some code.  There are ISO drivers that don't use
urb->next (like audio), but most video drivers do.  My patch #2
fixes one such driver.  My patch #3 fixes host controller drivers,
most of which were already converted.