lan78xx: Move enabling of EEE into PHY init code

Enable EEE mode as soon as possible after connecting to the PHY, and
before phy_start. This avoids a second link negotiation, which speeds
up booting and stops the interface failing to become ready.

See: https://github.com/raspberrypi/linux/issues/2437



Signed-off-by: Phil Elwell <phil@raspberrypi.org>