loader: drop return value for rom_add_blob_fixed

rom_add_blob never fails, and neither does rom_add_blob_fixed,
so there's no need to return value from it.

In fact, rom_add_blob_fixed was erroneously returning -1 unconditionally
which made the only system that checked the return value -M bamboo fail
to start.

Drop the return value and drop checks from ppc440_bamboo to
fix this failure.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
