hwmon: (it87) Avoid registering the same chip on both SIO addresses

IT8705F is known to respond on both SIO addresses. Registering it twice
may result in system lockups.

Reported-by: Russell King <linux@armlinux.org.uk>
Fixes: e84bd953 ("hwmon: (it87) Add support for second Super-IO chip")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>