hwmon: (max6650) Cache alarm_en register

The alarm_en register is read each time the is_visible function is called.
Since it is a configuration register, this is completely unnecessary.
Read it once and cache its value.

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