USB: serial: fix port release

We should not call kill_traffic (and usb_kill_urb) once disconnect
returns. Any pending urbs are killed at disconnect and new submissions
are prevented by usb_unbind_interface (and usb_disable_interface).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>