USB: cypress_m8: fix use-after-free in TIOCMIWAIT

USB: cypress_m8: fix use-after-free in TIOCMIWAIT

commit 356050d8b1e526db093e9d2c78daf49d6bf418e3 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.

Also remove bogus test for private data pointer being NULL as it is
never assigned in the loop.

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