sh_eth: add EDMR.NBST support

The R-Car V3H (AKA R8A77980) GEther controller adds the DMA burst mode bit
(NBST) in EDMR and the manual tells to always set it before doing any DMA.

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>