net: KS8851 needs to depend on MII

fix this build error when CONFIG_MII is not set
drivers/net/ks8851.c:999: undefined reference to `generic_mii_ioctl'
drivers/net/ks8851.c:1050: undefined reference to `mii_link_ok'
drivers/net/ks8851.c:1056: undefined reference to `mii_nway_restart'
drivers/net/ks8851.c:1044: undefined reference to `mii_ethtool_sset'
drivers/net/ks8851.c:1038: undefined reference to `mii_ethtool_gset'

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Acked-by: Ben Dooks <ben@simtec.co.uk>