net: rtl8169: Properly align buffers

RX and TX descriptor rings should be aligned to 256 byte boundaries. Use
the DEFINE_ALIGN_BUFFER() macro to define the buffers so that they don't
have to be manually aligned later on. Also make sure that the buffers do
align to cache-line boundaries in case the cache-line is higher than the
256 byte alignment requirements of the NIC.

Also add a warning if the cache-line size is larger than the descriptor
size, because the driver may discard changes to descriptors made by the
hardware when requeuing RX buffers.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>