enetc: Fix build error without PHYLIB

enetc: Fix build error without PHYLIB

If PHYLIB is not set, build enetc will fails:

drivers/net/ethernet/freescale/enetc/enetc.o: In function `enetc_open':
enetc.c: undefined reference to `phy_disconnect'
enetc.c: undefined reference to `phy_start'
drivers/net/ethernet/freescale/enetc/enetc.o: In function `enetc_close':
enetc.c: undefined reference to `phy_stop'
enetc.c: undefined reference to `phy_disconnect'
drivers/net/ethernet/freescale/enetc/enetc_ethtool.o: undefined reference to `phy_ethtool_get_link_ksettings'
drivers/net/ethernet/freescale/enetc/enetc_ethtool.o: undefined reference to `phy_ethtool_set_link_ksettings'
drivers/net/ethernet/freescale/enetc/enetc_mdio.o: In function `enetc_mdio_probe':
enetc_mdio.c: undefined reference to `mdiobus_alloc_size'
enetc_mdio.c: undefined reference to `mdiobus_free'

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: d4fd0404c1c9 ("enetc: Introduce basic PF and VF ENETC ethernet drivers")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>