hwmon: (nct6796) Clean up and amend fan/pwm configuration for NCT6796D

Now that everything is separated, clean up fan and pwm configuration
for NCT6796D. While doing that, take the forgotten configuration register
cre0 into account to determine if AUXFANIN2 (fan5) and AUXFANOUT2 (pwm5)
are connected.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>