USB: oti6858: fix abuse of interface data

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

Note that neither tiocmset or tiocmget need to check for disconnect as
they do not access the device.

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