USB: fix errornous assumption in the usb serial framework revealed by iuu_phoenix

the iuu_phoenix driver submits another URB from a completion handler.
This dictates a certain order of calls to usb_kill_urb() in kill_traffic().
As other drivers may do it the other way round, we need to use both
orders in kill_traffic().
This patch does so and should be merged before iuu_phoenix is merged.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>