net: sh_eth: clean up for the SH7757's code

The SH7757's ETHER can work using the SH7724's setting. So, the patch
modifies it.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>