usb-uhci: stop queue filling when we find a in-flight td

Not only QHs can form rings, but TDs too.  With the new
queuing/pipelining support we are following TD chains and
can actually walk in circles.  An assert() prevents us from
entering an endless loop then.

Fix is easy:  Just stop queuing when we figure the TD we are
about to queue up is in flight already.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
