net: phy: smsc: Implement PHY config_init for LAN87xx

The LAN8710/LAN8720 chips do have broken the "FlexPWR" smart power-saving
capability. Enabling it leads to the PHY not being able to detect Link when
cold-started without cable connected. Thus, make sure this is disabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christian Hohnstaedt <chohnstaedt@innominate.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>