net: sh_eth: remove __flush_purge_region

It is a function of SuperH architecture. There is no good to use
the function on a driver generally. So, the driver uses
dma_map_single() instead of __flush_purge_region.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>