USB: mct_u232: fix broken close

Fix regression introduced by commit
f26788da (USB: serial: refactor generic
close) which broke driver close().

This driver uses non-standard semantics for the read urb which makes the
generic close function fail to kill it (the read urb is actually an
interrupt urb and therefore bulk_in size is zero).

Reported-by: Eric Shattow "Eprecocious" <lucent@gmail.com>
Tested-by: Eric Shattow "Eprecocious" <lucent@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>