powerpc: Add missing error check to prom_find_boot_cpu()

prom_init.c calls 'instance-to-package' twice, but the return
is not checked during prom_find_boot_cpu(). The result is then
passed to prom_getprop(), which could be PROM_ERROR. Add a return check
to prevent this.

This was found on a pasemi system, where CFE doesn't have a working
'instance-to package' prom call.

Before Commit 5c0484e2 ('powerpc: Endian safe trampoline') the area
around addr 0 was mostly 0's and this doesn't cause a problem. Once the
macro 'FIXUP_ENDIAN' has been added to head_64.S, the low memory area
now has non-zero values, which cause the prom_getprop() call
to hang.

mpe: Also confirmed that under SLOF if 'instance-to-package' did fail
with PROM_ERROR we would crash in SLOF. So the bug is not specific to
CFE, it's just that other open firmwares don't trigger it because they
have a working 'instance-to-package'.

Fixes: 5c0484e2 ("powerpc: Endian safe trampoline")
Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/prom_init.c

@@ -2834,6 +2834,9 @@ static void __init prom_find_boot_cpu(void)
 
 	cpu_pkg = call_prom("instance-to-package", 1, 1, prom_cpu);
 
+	if (!PHANDLE_VALID(cpu_pkg))
+		return;
+
 	prom_getprop(cpu_pkg, "reg", &rval, sizeof(rval));
 	prom.cpu = be32_to_cpu(rval);