powerpc/sstep: Return directly after a failed address_ok() in emulate_step()

Setting err and going to ldst_done just returns 0, without using err, so
just return 0 directly. We already do that for other call sites in this
function.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[mpe: Rewrite change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>