This patch simplies the way the UHCI driver handles short control
transfers.  When a transfer is short the HC will stop handling that
endpoint, and it's necessary to get it going again so that the status
stage of the control transfer can take place.  Currently the driver does
this by allocating a new QH for the transfer and setting its element
pointer to point at the final status TD.  The old QH is recycled.  But
it's not necessary to go to all that trouble; the element pointer in the
original QH can be updated directly.

Normally the element pointer is supposed to be owned by the HC, and it's
not safe to just change its value since the HC may overwrite it at any
time.  But when a transfer is stopped because of a short packet, the
current TD is marked inactive and the HC will not update the element
pointer.  To write an unchanged pointer value back to memory would be a
waste of PCI bus cycles.  Now the UHCI spec doesn't say explicitly that an
HC _can't_ do this, but I've tested both Intel and VIA hardware and
neither of them does.

As a side effect of this change, some of the code for removing QHs can be
eliminated.