Fix abuse of interface data which was used to signal device disconnect.

Use the usb_serial disconnect flag and mutex where appropriate.

Note that there's no need to grab the mutex in chase_port as it does not
access the device.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>