powerpc/pmac/windfarm: Don't test pointers before kfree()

Fix minor nits found by cppcheck

[./macintosh/windfarm_pm81.c:760]: (style) Redundant condition. It is safe to deallocate a NULL pointer
[./macintosh/windfarm_pm81.c:762]: (style) Redundant condition. It is safe to deallocate a NULL pointer

Signed-off-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>