USB: mct_u232: clean up read implementation

The device uses the second interrupt-in endpoint of the interface for
reading. Stop abusing the port read urb and store a pointer to the
second interrupt-in urb as port-private data instead.

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