sh_eth: remove duplicate sh_eth_set_duplex() definitions

Remove all the duplicate definitions of sh_eth_set_duplex() under  different
#ifdef's, leaving only one outside the #ifdef's. We have to annotate it with
'__maybe_unused' since it's called not from all SoC #ifdef blocks.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
[Sergei: annotated sh_eth_set_duplex() as '__maybe_unused', added the changelog,
reworded the subject, changing the prefix.]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>