USB: oti6858: do not call set_termios with uninitialised data

Make sure set_termios is not called with uninitialised data at open. The
old termios struct is currently not used, but pass NULL instead to avoid
future problems (e.g. stack data leak).

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