drivers: ethernet: nxp: fix mac addr order

make sure, that zephyr,random-mac-address
has a higher prio, than local-mac-address, as
documented in ethernet-controller.yaml.

Also make sure, that no vaild mac address
doesn't lead to init fail, as it can still be
set later via set_config.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>