net: sh_eth: use miiphybb instead of own mii functions

The sh_eth driver had an own mii functions. However the function
didn't support the gigabit PHY. The U-Boot has the general phy driver
and miiphybb driver, and they already support it.
So this patch removes the own functions and uses the phy driver.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>