net: mii: make mii_ethtool_gset() return void

mii_ethtool_gset() does not return any errors. Since there are no users
of this function that rely on its return value, it can be
made void.

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>