hwmon: (max6639) Fix FAN_FROM_REG calculation

RPM calculation from tachometer value does not depend on PPR.
Also, do not report negative RPM values.

Signed-off-by: Chris D Schimp <[email protected]>
[[email protected]: do not report negative RPM values]
Signed-off-by: Guenter Roeck <[email protected]>
Cc: [email protected] # 3.0+
Acked-by: Roland Stigge <[email protected]>
1 file changed