hwmon: (max6650) Simplify alarm handling

Instead of re-reading the alarm register after reporting an alarm,
mark cached values as invalid. While this results in always reading all
data on subsequent reads, it is quite unlikely that such reads will
actually happen before the cache times out. The upside is avoiding
unnecessary unconditional i2c read operations.

Cc: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>