Revert "net: stmmac: enable multiple buffers"

The commit aff3d9ef

 ("net: stmmac: enable multiple buffers") breaks
numerous boards. while some patch exists for fixing some of it,
dwmac-sunxi is still broken with it.
Since this patch is very huge, it will be better to split it in smaller
part.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>