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

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

[ Upstream commit f6de298806d9cbc63a4907bca34a06162b9d7dce ]

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: 81820059a428 ("hwmon: (nct6775) Add support for NCT6796D")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>