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>

drivers/macintosh/windfarm_rm31.c

@@ -439,15 +439,15 @@ static void backside_setup_pid(void)
 
 /* Slots fan */
 static const struct wf_pid_param slots_param = {
-	.interval	= 5,
-	.history_len	= 2,
-	.gd		= 30 << 20,
-	.gp		= 5 << 20,
-	.gr		= 0,
-	.itarget	= 40 << 16,
-	.additive	= 1,
-	.min		= 300,
-	.max		= 4000,
+	.interval	= 1,
+	.history_len	= 20,
+	.gd		= 0,
+	.gp		= 0,
+	.gr		= 0x00100000,
+	.itarget	= 3200000,
+	.additive	= 0,
+	.min		= 20,
+	.max		= 100,
 };
 
 static void slots_fan_tick(void)