commit 8bcd4e6a upstream.

Make sure to use a properly refcounted tty_struct in write_wake up to
avoid dereferencing a NULL-pointer when a port is being hung up.

Fixes: bed35c6d ("serdev: add a tty port controller driver")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>