hwmon: (nct6775) Fix RPM output for fan7 on NCT6796D

fan7 on NCT6796D does not have a fan count register; it only has an RPM
register. Switch to using RPM registers to read the fan speed for all
chips supporting it to solve the problem for good.

Reported-by: Robert Kern <ulteq@web.de>
Cc: Robert Kern <ulteq@web.de>
Fixes: 81820059 ("hwmon: (nct6775) Add support for NCT6796D")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>