USB: fix race leading to use after free in io_edgeport

usb_unlink_urb() is asynchronous, therefore an URB's buffer may not
be freed without waiting for the completion handler. This patch switches
to usb_kill_urb(), which is synchronous.
Thanks to Alan for making me look at the remaining users of usb_unlink_urb()


Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Al Borchers <alborchers@steinerpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>