The aspeed-pwm-tacho controller supports measuring the fan tach by using
leading, falling, or both edges.  This change allows the driver to
support either of the three configurations and will appropriately modify
the returned tach data.

If the controller is measuring with both edges it can return a value more
quickly to the requestor.  This version of the driver should still take ~1s
to return with an RPM value per fan, however, it can be tuned faster with
double edge counting enabled than without.

I tested this and found the number returned matched what I expected.
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>