powerpc/iseries: Kexec is known not to work on iseries

The non-zero return from the prepare callback is returned by sys_kexec_load()
to userspace, indicating that kexec is not supported on the machine.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>