powerpc/windfarm: fix compiler warning

Fixes this warning:

drivers/macintosh/windfarm_pm91.c: In function 'wf_smu_cpu_fans_tick':
drivers/macintosh/windfarm_pm91.c:237:2: warning: passing argument 1 of 'wf_sensor_get' from incompatible pointer type
drivers/macintosh/windfarm.h:124:19: note: expected 'struct wf_sensor *' but argument is of type 'struct wf_sensor **'

Introduced by commit 33e6820b ("powerpc/windfarm: Add useful
accessors").
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/macintosh/windfarm_pm91.c

@@ -234,7 +234,7 @@ static void wf_smu_cpu_fans_tick(struct wf_smu_cpu_fans_state *st)
 		return;
 	}
 
-	rc = wf_sensor_get(&sensor_cpu_power, &power);
+	rc = wf_sensor_get(sensor_cpu_power, &power);
 	if (rc) {
 		printk(KERN_WARNING "windfarm: CPU power sensor error %d\n",
 		       rc);