spapr: add missing break in h_get_cpu_characteristics()

Detected by Coverity (CID 1385702). This fixes the recently added hypercall
to let guests properly apply Spectre and Meltdown workarounds.

Fixes: c59704b2 "target/ppc/spapr: Add H-Call H_GET_CPU_CHARACTERISTICS"
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>