powerpc/powernv: Bail upon invalid master PE

When freezing compound PEs in pnv_ioda_freeze_pe(), we should bail
upon illegal master PE. We needn't freeze slave PE because it should
have been put into frozen state by hardware.

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>