hwmon: (lm90) Mark alert as broken for MAX6680

Experiments with MAX6680 and MAX6681 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>