powerpc/powernv/opal-dump : Handles opal_dump_info properly

Moves the return value check of 'opal_dump_info' to a proper place which
was previously unnecessarily filling all the dump info even on failure.

Signed-off-by: Mukesh Ojha <mukesh02@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>