ppc/pnv: check for OPAL firmware file presence

and exit before uselessly trying to load it if the file does not
exists.

Issue discovered by Coverity Scan.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>