sh_eth: remove sh_eth_cpu_data::rpadir_value

If RPADIR exists, the value written to it is always the same for all SoCs
(and derived from NET_IP_ALIGN), so there has not  been any need to store
it in the *struct* sh_eth_cpu_data...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>