mpc85xx: correct TP-LINK TL-WDR4900 MAC addresses

mpc85xx: correct TP-LINK TL-WDR4900 MAC addresses

This commit fixes TP-Link TL-WDR4900 v1 MAC address assignment.
Previously, the MAC addrss was read for the ethernet from the "config"
partition. However, the content of this partition is dependent on the
firmware which was previously installed on the device.

Switch the MAC address source to the U-Boot partition, where the MAC
address is always present at a fixed partition. The partition was
previously already used for the WiFi MAC-addresses.

Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: David Bauer <mail@david-bauer.net>