Make sure the interrupt urb submitted in port_probe is killed in
port_remove.

The interrupt-urb completion handler references the port and may get
called after port_remove has returned and the port has been
unregistered (although this is currently prevented by usb-serial core as
we are using a non-private urb).
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>