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

Since commit 698a7c24a544 ("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 <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>

drivers/hwmon/nct6775.c

···
 	if (err)
 		return err;
 
-	if (force_id)
+	val = (superio_inb(sioaddr, SIO_REG_DEVID) << 8) |
+		superio_inb(sioaddr, SIO_REG_DEVID + 1);
+	if (force_id && val != 0xffff)
 		val = force_id;
-	else
-		val = (superio_inb(sioaddr, SIO_REG_DEVID) << 8)
-		    | superio_inb(sioaddr, SIO_REG_DEVID + 1);
+
 	switch (val & SIO_ID_MASK) {
 	case SIO_NCT6106_ID:
 		sio_data->kind = nct6106;