arm: mvebu: turris_omnia: Overwrite ethaddr only if invalid

Currently we always overwrite ethaddrs with those from EEPROM.

In order to allow user to use a cloned MAC address in U-Boot, change the
code so that it sets ethaddr variables only if they aren't set or are
invalid.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>