powerpc: Fix default_machine_crash_shutdown #ifdef botch

powerpc: Fix default_machine_crash_shutdown #ifdef botch

commit c2be05481f6125254c45b78f334d4dd09c701c82 upstream.

crash_kexec_wait_realmode() is defined only if CONFIG_PPC_STD_MMU_64
and CONFIG_SMP, but is called if CONFIG_PPC_STD_MMU_64 even if !CONFIG_SMP.
Fix the conditional compilation around the invocation.

[PG: commit b3df895aebe091b1657 "powerpc/kexec: Add support for FSL-BookE"
 introduced the PPC_STD_MMU_64 check this refers to, but we don't want to
 cherry pick that whole addition to stable just for one #ifdef line.]

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>