hwmon: (it87) Preserve configuration register bits on init

commit 41002f8dd5938d5ad1d008ce5bfdbfe47fa7b4e8 upstream.

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>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>