USB: usbtest fix endless loop in unlink tests.

In tests 11 and 12 if the URB completes with an error status (eg babble)
the asynchrous unlink entered an endless loop trying to unlink
a non resubmitted URB.

Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>