hwmon: (nct6775) Fix fan6 detection for NCT6793D

Commit 2d99925a ("hwmon: (nct6775) Separate fan/pwm configuration
detection for NCT6793D") accidentally removed part of the code detecting
if fan6 is enabled or not. As result, fan6 is no longer detected on Asus
PRIME Z370-A. Restore the missing detection code.

Fixes: 2d99925a ("hwmon: (nct6775) Separate fan/pwm configuration detection for NCT6793D")
Reported-by: Chris Siebenmann <cks@cs.toronto.edu>
Cc: Chris Siebenmann <cks@cs.toronto.edu>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>