fancontrol: Fix shell error when FCFANS is not set

The bug was introduced when I added support for multiple fans
controlled by the same PWM output, in r6000. lm-sensors versions
affected: 3.3.2 to 3.3.5.

This fixes SUSE bug #903520:
https://bugzilla.suse.com/show_bug.cgi?id=903520
Add detection of Nuvoton NCT6792D