sh_eth: remove '__maybe_unused' annotations

Now that  the SoC specific support is no longer done with help of #ifdef'fery,
we  no longer need '__maybe_unused' annotations to sh_eth_select_mii() and
sh_eth_set_duplex()...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>