arm: mvebu: turris_mox: Better check for valid ethernet addresses in env

Currently we overwrite ethaddr and eth1addr only if these variables
don't exist.

Better overwrite them even if the env variable exists, but is invalid -
eth_env_get_enetaddr_by_index() checks for validity.

Refactor the code to use a for cycle.

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