hwmon: (nct6775) Use different register to get fan RPM for fan7

The documented register to retrieve the fan RPM for fan7 is found
to be unreliable at least with NCT6796D revision 3. Let's use
register 0x4ce instead. This is undocumented for NCT6796D, but
documented for NCT6797D and NCT6798D and known to be working.

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>