A lot of people with USB controllers made by VIA have been suffering from
the fact that these controllers stop working when they receive a babble
packet.  In particular, they stop generating interrupt requests.  Since
the UHCI driver relies on IRQs from the controller for proper timing and
interlocking of unlink requests, this means that those broken controllers
will hang the UHCI driver and drivers for any device connected through it.

This patch, written by Herbert Xu, gives the UCHI driver the ability to
manage the unlink procedure using timer interrupts as well as controller
interrupts.  (It also fixes a race in the UHCI irq handler.)  Although it
won't prevent the VIA controllers from flaking out when they encounter
babble, at least now users will be able to rmmod the uhci-hcd driver and
then reload it, restoring their systems back to normal operation (until
the next babble!).

P.S.: Herbert, there's one loose end I still want to tie up.  When the
controller isn't running (i.e., is suspended) the frame number won't
change, but unlinks still need to work.  It's a small point, not too
likely to come up in normal usage.  I'll fix it later on when I update the
state-changing part of the driver.