USB: serial: use urb poison to reliably kill traffic

Use usb_poison_urb to reliably kill all urbs on disconnect and suspend.

This way there will be no question that the urbs cannot be resubmitted
by buggy subdrivers.

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