i8k: Register only temperature sensors which have labels

Detect presense of sensor by calling type function instead trying to read
temperature value. Type function is working also for sensors which are temporary
turned off (e.g on GPU which is turned off). Dell DOS binary NBSVC.MDM is doing
similar checks, so we should do that too.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>