net: sh-eth: Add cache writeback control after setting bit of DMA descriptor

A bit field of DMA descriptor is on memory area of DMA. If we set data
to bit of DMA descriptor, we must be writeback data of DMA descriptor.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>