rtl8139: use ldl/stl wrapper for unaligned 32-bit access

The tx offload feature accesses a 16-bit aligned TCP header struct.  The
32-bit fields must be accessed using ldl/stl wrappers since some host
architectures fault on unaligned access.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1438604157-29664-4-git-send-email-stefanha@redhat.com