net: asix: fix operation without eeprom

This patch fixes operation of our on-board AX88772B chip without EEPROM
but with a ethaddr coming from the regular U-Boot environment. This is
a forward port of some remaining parts initially implemented by
Antmicro.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Marek Vasut <marex@denx.de>