powerpc: Removing undead ifdef __KERNEL__

The __KERNEL__ ifdef isn't necessary at this point, because it is
checked in an outer ifdef level already and has no effect here.

Signed-off-by: Christian Dietrich <qy03fugy@stud.informatik.uni-erlangen.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>