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 <silverchris@gmail.com>
[guenter.roeck@ericsson.com: do not report negative RPM values]
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: stable@vger.kernel.org # 3.0+
Acked-by: Roland Stigge <stigge@antcom.de>