This patch is needed to make high bandwidth ISO streams behave,
but could resolve some other scanning glitches.  Current users
of periodic transfers (interrupt transfer modes for hubs, mice,
and keyboards) shouldn't even notice this change.

It makes the periodic schedule scan handle cases where a given
frame's schedule slot reports completions in several different
microframes.  So far that's been uncommon, but it's typical
for high bandwidth iso (or even with busier interrupt trees than
this driver has supported yet).

It also starts to remove the assumption that each ITD only uses
one microframe; but most of those changes are in the next patch.
And it fixes a bug where some status bits were mis-interpreted as
significant bits in the ITD transfer length.