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 33e6820b767a ("powerpc/windfarm: Add useful
accessors").

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>