hwmon: (max31790) Fix fan speed reporting for fan7..12

Fans 7..12 do not have their own set of configuration registers.
So far the code ignored that and read beyond the end of the configuration
register range to get the tachometer period. This resulted in more or less
random fan speed values for those fans.

The datasheet is quite vague when it comes to defining the tachometer
period for fans 7..12. Experiments confirm that the period is the same
for both fans associated with a given set of configuration registers.

Fixes: 54187ff9 ("hwmon: (max31790) Convert to use new hwmon registration API")
Fixes: 195a4b42 ("hwmon: Driver for Maxim MAX31790")
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Václav Kubernát <kubernat@cesnet.cz>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210526154022.3223012-2-linux@roeck-us.net

drivers/hwmon/max31790.c

@@ -170,7 +170,7 @@ static int max31790_read_fan(struct device *dev, u32 attr, int channel,
 
 	switch (attr) {
 	case hwmon_fan_input:
-		sr = get_tach_period(data->fan_dynamics[channel]);
+		sr = get_tach_period(data->fan_dynamics[channel % NR_CHANNEL]);
 		rpm = RPM_FROM_REG(data->tach[channel], sr);
 		*val = rpm;
 		return 0;