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

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>