powerpc/windfarm: Fix noisy slots-fan on Xserve (rm31)

slots-fan on G5 Xserve is always running at full speed with windfarm_rm31
driver, resulting in a very high acoustic noise level. It seems the fan
parameters are incorrect, and have been copied from the Drive Bay fan
(RPM, not present on rm31) of the legacy therm_pm72 driver. This patch
changes the parameters to match the Slots fan (PWM) of therm_pm72. With
the patch, slots-fan speed drops from 99% to 19% during normal use,
and slots-temp settle to ~42'C.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
CC: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>