hwmon: (nct6775) Do not accept force_id unless chip is found

Since commit 698a7c24 ("hwmon: (nct6775) Support two SuperIO chips
in the same system"), the driver supports two Super-IO chips. This has
the undesirable side effect that force_id always detects a second chip
at address 0xfff8, even if no chip exists at that address.

nct6775: Found NCT6793D or compatible chip at 0x4e:0xfff8

If no chip at all is found at a given SIO address, it does not make sense
to instantiate it. Limit force_id to only work if some chip is found,
that is if the chip ID returns a value other than 0xffff.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>