target-ppc: fix index array of national digits

Fixes the big endian array access of national digits, from commits
b8155872 and e2106d73.

Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>