USB: mct_u232: fix use-after-free in TIOCMIWAIT

USB: mct_u232: fix use-after-free in TIOCMIWAIT

commit cf1d24443677a0758cfa88ca40f24858b89261c0 upstream.

Use the port wait queue and make sure to check the serial disconnected
flag before accessing private port data after waking up.

This is is needed as the private port data (including the wait queue
itself) can be gone when waking up after a disconnect.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>