hwmon: (max31790) Detect and report zero fan speed

If a fan is not running or not connected, of if fan monitoring is disabled,
the fan count register returns a fixed value of 0xffe0. So far this is then
translated to a RPM value larger than 0. Since this is misleading and does
not really make much sense, report a fan RPM of 0 in this situation.

Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Václav Kubernát <kubernat@cesnet.cz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Václav Kubernát <kubernat@cesnet.cz>
Link: https://lore.kernel.org/r/20210526154022.3223012-7-linux@roeck-us.net