USB: mos7840: fix tiocmget error handling

Make sure to return errors from tiocmget rather than rely on
uninitialised stack data.

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