USB: serial: fix generic disconnect implementation

There is no need for the generic disconnect callback to stop the read
and write urbs a second time as this has already been taken care of by
close (which is called from hangup as part of disconnect).

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