USB: mct_u232: fix broken close

Make sure generic close is called at close.

The driver relies on the generic write implementation but did not call
generic close.

Note that the call to kill the read urb is not redundant, as mct_u232
uses an interrupt urb from the second port as the read urb and that
generic close therefore fails to kill it.

Compile-only tested.

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