target-arm: Fix aarch64 disas_ldst_single_struct

We add s->be_data within do_vec_ld/st.  Adding it here means that
we have the wrong bits set in SIZE for a big-endian host, leading
to g_assert_not_reached in write_vec_element and read_vec_element.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-id: 1481085020-2614-3-git-send-email-rth@twiddle.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>