hwmon: (lm90) Mark alert as broken for MAX6646/6647/6649

commit f614629f upstream.

Experiments with MAX6646 and MAX6648 show that the alert function of those
chips is broken, similar to other chips supported by the lm90 driver.
Mark it accordingly.

Fixes: 4667bcb8 ("hwmon: (lm90) Introduce chip parameter structure")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>