hwmon: (it87) Preserve configuration register bits on init

We were accidentally losing one bit in the configuration register on
device initialization. It was reported to freeze one specific system
right away. Properly preserve all bits we don't explicitly want to
change in order to prevent that.

Reported-by: Stevie Trujillo <stevie.trujillo@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>